Mechanical supplies:
2x spare inner tubes
Tyre levers
Puncture repair kit (for the extremes)
Mini pump
Multi-tool (w/ chain tool)
Short length of chain and/or a quick link
Food stuffs:
A few slices of malt loaf or granola bar (eat one every 20 mile or so)
800ml-1ltr of fluid (you can get refill's at a feed station or if needed, when passing a shop)
A handful of jellybabies (eat one every now and then between larger food items)
A banana
Emergency:
A couple of quid in change to use a payphone or buy an emergency snack or something.
An emergency contact number stashed somewhere on you.