First thing you need is a properly working bicycle, preferably light and fast, with good gears and good brakes, reflectors, lights and a strong rear rack. Please check your bike at home for old, worn or broken parts, so that you will not have to struggle with that on the road. We'll offer a bike repair and improvement workshop in London the day before we set off but still check your bike before leaving home because there won't be time for huge overhauls. Experience shows that people with racing bikes, whilst faster tend to need more maintenance than mountain-bikes, so a pre-trip service is especially important.
Pre trip maintenance checklist:
Some adjustments take a little trial & error, so it's worth doing your service in advance of the ride & taking the bike out for a spin, so you can be sure you've got it right. For further information on maintenance, "Richards Bicycle Book", Richard Ballantine is especially recommended.
A word on bicycle efficiency - if your mountain bike has knobbly off road tyres, get yourself a pair of "slicks" (road tyres), the thinnest ones that fit your rims will be ideal for autumn road cycling, make pedalling much easier and you less knackered!
Bring a small repair kit (at least some glue and patches), spare inner tube, extra spokes, tools, mudguards (friendly for the person riding behind you) and a bell, horn or a siren. A good lock! The UK is famous for its skilled bike thieves. Helmet.
Then you need waterproof panniers to store your stuff in. Better get the good ones (eg welded seems rather than sewn), as the cheaper versions tend to disappoint you sooner or later, or make panniers yourself. Also make sure that the method of attaching them to your bike is secure and that they aren't going to jump off when you go over bumps. It's also better for you if they don't take ages to fix to your bike since you will have to remove either them or their contents every night. Try to divide your luggage over the front and back of your bike, for better road holding up hills, or better still don't bring too much stuff.
Water - bottle carrier and bottle to carry a day's worth of water.
Food - snack food, plate/cup, spoon, knife, fork - but it's worth having your own basic culinary kit in case the group needs to separate at any point.
Personal equipment - take the minimum luggage as possible, since you have to carry it all by yourself & the more weight you have to pedal with, the more tired you'll be at the end of the day. But be sure to bring warm sweater, long trousers, T-shirt, shorts, socks, underwear, towel, toothbrush & paste, sandals, (people who can't sleep without a pillow could take just take the pillow bag and stuff it with clothes). Passport, visa, some money and the travel and health insurance-papers, as well as any medicines you might need!
Rain protection - raincoat, waterproof trousers (it can rain persistently in the UK), some strong waterproof bags big enough to pull over your shoes, spare shoes, trousers, shirt, waterproof bags for your luggage (or strong and big enough plastic bags to put over your luggage.). An airy shirt with long sleeves, scarf, gloves...
Other equipment (potentially shared with friends) - try to share equipment with friends to save weight (eg bike tools, mobile phone, camera, tent, cooking utensils, compass), mosquito repellent, alarm (watch/clock/mobile), 100% recycled toilet paper, torch, pocket knife, camera/video camera, notebook + pen, compass/GPS system (!), good maps (as opposed to water soluble)