So the original question is answered - it *is* possible to ride from Land's End to Lowestoft overnight on the night of the solstice.
While in Penzance, we checked the weather forecasts for Sunday and Monday nights. Monday, our original plan, was forecast snowfall for the entire night all along our journey, while the Sunday night was only sleety and haily. We decided that Sunday night was the better option.
Before the ride, Joel and I had different, but quite clear, ideas of what would be the biggest difficulty - I was only concerned with keeping warm, and Joel was more concerned about the prospect of not being able to see where he was going. Joel had therefore augmented his headlight with a pair of foglights, each of equal brightness to the original headlight, and wired them in rather neatly. He's also made himself a pair of nifty seven-way adjustable brackets for them.
I, meanwhile, packed enough luggage for a several week stay - the total luggage capacity of my bike is roughly equal to two decent sized backpacking rucksacks, and I'd filled them with warm (and occasionally waterproof) stuff. I'd also added a screen to my bike, and brought along some emergency heated grips.
In Penzance, I noticed that Joel had a point, and scurried off to Halfords where I bought the first things that looked like they'd fit, and rather than fashion a nice and stable bracket for them, cable-tied them onto my indicators. I spent rather a long time connecting them up, repeatedly trying to work out why there was no power available while not checking whether I'd blown the sensibly included fuse. I think Joel meanwhile reasoned that I had enough excess warm clothing to solve all of Crisis' problems, so he'd just nick some of mine if it was necessary. Though he did bring along a tent.
Thanks in no small part to my ignorance of the fuse we didn't leave Penzance until it was already dark. We got to Land's End for about 5:30, dawdled for a bit, and then left for Exeter.
On the way up to Exeter the piles of grit along the edge of the road rather quickly turned into piles of snow which wasn't exactly what we'd been hoping for - I'd inferred from the weather forecast that we'd not see snow until at least after the first stop. We pulled in to Whiddon Down services for dinner in the Little Chef. There was snowfall on the petrol station forecourt as we filled up, and the Little Chef car park was effectively a sheet of ice. We parked on the petrol station forecourt, and skated off to interrupt the waiters' game of football and fashion some duct-tape-and-bin-bag overshoes.
Leaving Exeter, Joel pulled over to readjust his headphones and when we went to leave I found I couldn't start my engine. On hitting the start button, the starter motor turned, the lights went dim and then nothing happened, which is pretty certainly down to a flat battery. I whipped out a multimeter to Joel's apparent surprise, and found that the battery was indeed flat. Fortunately, I've had this bike for some time, so there is a lot of wiring all over it that isn't doing much any more, and from this and a screwdriver we fashioned a set of jump leads. After some acrobatics getting the bikes close enough (I didn't have any long spare wires), the bike came to life again. I tested the charging circuit and found it was at 13V at 5000rpm, so the battery should have been charging. We assumed I'd been running the engine too slowly, which is a long-standing habit of mine, and causes not enough power to be generated to charge the battery.
Stonehenge was dark and cold. We stopped at the side of the road where we suspected Stonehenge would be - there were VW campers parked up in an otherwise attraction-free layby - and Joel took it upon himself to get a photo of him and the stones, while I attempted to coax my feet out of anesthesia. At the time, doing this while standing in several inches of snow didn't seem as counter-productive as it does in hindsight.
The stones were closed, and they even had security guards wandering round keeping people out. Apparently if we waited until dawn they'd let us in, though we mused that we'd arrived at midnight, which must be a significant time to someone.
In the meantime, I checked the temperature on the thermometer on my handlebars, which was reading -5.4 as we pulled up, and to my delight found that it had settled on a decidedly more humane -3.5. In celebration Joel cracked out the coffee and biscuits and demanded we stop somewhere with walls and a roof as soon as possible. Apparently he rather likes the ability to feel his feet.
So, we cheated and found ourselves on the M3 at about 1am. Winchester services were amazing. Firstly, I discovered that my charging circuit and battery were apparently fine - they just couldn't cope with my extra lighting - so I resolved to stop using the extra lights about 20 minutes before any stop to give the battery a chance to charge to the point where it'd be able to start the engine. Secondly, it had heating. I didn't realise how cold I was until I sat down and shook for a few minutes. Joel rudely disturbed the woman in the Costa shirt by asking her if she wouldn't mind making us a pair of coffees, but the guy in WHSmith was quite amused by the idea of company, and repeatedly told us it was a stupid idea.
I did some calculations as to how far on or off schedule we were (I asked the satnav), and found that if we went non-stop up the motorway, through London, and up the A12, we'd get there at abut 6:50am, with dawn happening from about 7:30 to 8:30. With this, and the fact it was jolly cold outside so we'd want to stop, in mind, we set off up the M3.
London was weird, it was the first real traffic we'd seen in about 500 miles and two days, and while feeling like home, we were still about 150 miles from the end. Though by the time we got to Trafalgar square, I found I'd fallen into the mindset that we'd pretty much finished.
We got into London at about 3:30am, and were at Trafalgar Square at 4, as London was waking up in its very businesslike manner, surrounded by festivities. We left London knowing we had just to go up the A12 which be both knew as a far as Ipswich, and then some, and so we should be at Lowestoft well within three hours.
So we left London in good spirits. The first 50 miles or so weren't too bad; we were riding through misty rain on unlit roads with a whole lot of traffic coming in the other direction with maladjusted headlights. In short, I could generally see to just ahead of my headlight, sometimes even as far the vehicle in front. It was about here that I realised how lucky we'd been with the traffic and the weather so far.
Since it was a Sunday night and not much would be open, we'd agreed to pull into every open petrol station, on the grounds that several quick stops would be preferable to none at all. The first stop was pretty icy, but usable. I stocked up on those little energy shots (don't get the Lucozade one, it's vile), and we cleaned visors and headlights.
From there, it pretty much went downhill. Every petrol station was iced over and/or closed until Ipswich, at which point I'd decided the massive Tesco Extra would be pressed into service as a stop. That, too, was icy, so we pressed on.
And then we ran out of grit. From Ipswich, no-one had deigned to grit the roads. We were on the dual-carriageway A12 and suddenly the overtaking lane disappeared under several inches of snow, and shortly afterwards a line of snow appeared down the middle of the remaining lane. This was apparently fine for the car drivers who were behind us, but on a bike you can't really afford to just slip a bit on one wheel, so we were down to about 30mph on a national speed limit dual carriageway. I decided we'd pull over the next time it was sensible, and do whatever we could to make this more humane.
About 5 miles later, the dual carriageway became a single carriageway, which meant that the steel central reservation was replaced with a snow one and the track we had through the snow was replaced with a strip of shallower slush about the width of a car down the middle of the road. By now, I couldn't feel my feet or the footpegs, and instead of nudging the gear changer with my toes I was stomping on it with my heel, with only a vague idea of where it was relative to my feet. I was keeping an eye on Joel in my wing mirrors, since we'd agreed that he'd flash his lights if he wanted to stop and wasn't in a position to overtake and pull over, but from when the countdown on the satnav ticked over to 20 miles to Lowestoft, I think I stopped watching most things, and was overcome with quite some determination to just get to the finish, and go somewhere warm and dry.
That 20 miles wasn't really indescribably bad, but it's difficult to make it sound realistic. So, as a substitute: it was ruddy painful, cold and petrifying and I've never been so happy to see a wind turbine as I was when we arrived at Ness Point. There never was a point to pull over, sensible or otherwise.
We got to Ness Point as dawn was breaking, where our dad was to meet us with a car and a trailer to offer us a lift home. We pulled up and he wasn't there, apparently he'd seen the 400m or so of sheet ice leading up to it and thought we wouldn't be so stupid as to have ridden along it. I'd not given it a second thought.
I rang him, and he arrived and took us to have breakfast at the Asda cafe. At 7:30 am in Lowestoft, there didn't appear to be much open, and we weren't in the mood for looking. After amusingly managing to give us each exactly not what we'd ordered, we went back to the bikes to load the trailer.
After some to-ing and fro-ing working out which bike was to go where and how (Joel's tyres were antisocially wide), we got the bikes on and strapped down, just as the left tyre of the trailer went flat. Fortunately, there were a couple of guys from the local council in a van, apparently with some time to kill, who offered us a jack to help get the wheel off. They then took dad off to the local Kwik Fit to get the tyre changed while Joel and I strapped the bikes down and tried to avoid noticing the salt they were covered in.
Dad got back with a wheel, fitted it, and we had a delightfully warm, eventless, warm, comfortable, warm, dry and warm drive back through several hours of traffic. I was pretty happy to have taken him up on the offer of the trailer. And warm.