Done it twice, both as teams but never solo/pairs.
If I did any race like it again I would definitely take every piece of cycle clothing you've got. The two years I did it were luckily dry so we didn't get drenched. Even so sitting around waiting for your next lap in sweaty kit not pleasant. Nice dry clothes at the start of every lap were great.
As already said above, warm clothes for between laps.
In general I'd take anything you'd take if you were going camping: tent, inflatable matress, ear plugs, (to block the drone of generators/snoring), gazebo, beer, stove to make hot drinks, camping chairs, lighting so you can see the hot drink your making, bog roll, cold box for food, hip flask, bin bags.
And if you have it, everything you'd take if you were off on a riding trip. Bike, workstand, tools, spares, track pump, bucket and sponge (to wash bike if muddy between laps)
No idea if there water available at High lodge, but we always had big containers of water as there was no water on site in May Day arena. Info such as this should be on the web site which is usually pretty good.
Also there's a Tesco about 10 mins drive from the site, so there's always chance to stock up on last minute food supplies on the day.