As Mike says, it depends entirely where you're going, but it also depends on you. Some people always take chocolate/ food underground, others (myself generally included) prefer to carry as little as possible. So if I were you, I'd ask the trip leader, as they will know where you are going and you can discuss your own requirements.
As a minimum, I'd bring a spare light. My recommendation is for this to be separate to your helmet, so if for any reason you loose your helmet (it can happen), you still have light. I keep a Petzl Pixa around my neck, pretty much all the time. Other's have other ideas, see here for some thought: https://ukcaving.com/board/index.php?threads/backup-light.27442/
Other things to think about:
- How long is the walk to the cave?
On a hot day it's can be worth bringing a water bottle for the walk to/ from the cave.
- Is there water in the cave and is it safe to drink?
If not, consider bringing water.
- How long will you be underground?
If it's only a few hours, you may decide not to take anything, if it's a 16 hour epic, you might want a bit more. This could include food, water, flask of coffee, camping stove, first aid kit, shelter, knife, pulley.
- How awkward is the cave/ how much kit are you already carrying?
If the cave's pretty easy going, then you might not worry about carrying a bit of extra kit. If it's a really awkward trip, you may want to keep things to a minimum.
If you're already carrying 6 bags of rope or camera equipment, carrying a bit of food won't add much, but if you don't need any kit at all, carrying a bag just for food will slow you down.
Don't bring a banana if you're expecting a lot of flat out crawling. Tiffin or flapjack is pretty robust. Same applies to KitKat vs Double Decker...
- Are you going in and out the same way?
If yes, you can leave car keys etc at the entrance (maybe put them in a bag to keep them dry). If you're doing a through trip best not do that!
- Is the cave pretty?
Maybe bring a camera. For underground photography (speaking as a total amateur), I find a mini tripod makes a huge difference.
- What's the best way to carry your spare stuff?
Most oversuits have a pocket or two, but stuff in chest pockets can be a problem in a narrow cave...
If you're already brining a bag, a lot have a small pocket inside suitable for a snack.
A small tackle bag can be a good option and can be clipped toy your belt (Warmbac make a variety of sizes that you can buy from any good caving shop)
For longer/ wetter trips, you might want to carry things in a plastic drum. I'm a fan of the "BDH" chemical container, but they are pretty hard to find. More readily available from any decent caving shop, or 2nd hand online is the "Daren Drum" that comes in 2 convenient sizes. If you're feeling flush, or carrying expensive kit, Peli cases are worth their weight in gold.
I'd avoid putting anything in your bra, as it will probably be quite uncomfortable if the cave gets narrow, but I have no experience in this department!
Depending on your helmet, you may be able to stash a space blanket or bag of pork scratchings in the cradle.
A mini dry-bag can be a good addition, especially if you're bringing flapjack.
- Do you smoke?
Possibly a slightly contentious one, but it depends where you are and who you are with. I'm not a smoker, but I cave with smokers and have no problem if they spark up underground, so long as there is a decent draught and it's blowing away from me. Make sure to keep them dry though!
Finally, it's a good idea to bring a watch!