Hi I don't know how different the situation is in the UK than here but I can at least share my experience
I used to go camping in the GA mountains
If there is a source of running water nearby like a creek, you don't need to bring bottled water. We have always used small pumps that strain out all dangerous bacteria in the water or they also sell mineral tablets that will help that. We wash our dishes with water that's gone through the pump.
Bring a bit of toilet paper/paper towels to help clean up
Make sure you have a way to dispose of garbage, if there are no trash cans near the site then just bring some thick garbage bags to bring trash with you to throw away
Bring or dress in layers because it can get cold at night
Make sure you have extra matches and a lighter in case one doesn't work, a flash light, bandaids and alcohol for any accidents
Biodegradable soap such as Dr. Bronner's, good to wash cuts or wash your hands or dishes without harming the water
Sleeping bags
Weather-proof tent
Pot for cooking
Pocketknife
Canned and dried foods, plastic plates (washable or disposable) for eating
For fun of course your need marshmallows, it's fun to just grab a stick, clean it up with a knife blade and roast over a bon fire
We like to actually catch the marshmallows on fire, let them burn for a few minutes then blow them out
They're HOT but the outside is charred and the insides are gooey
IMO you really don't need that much to have a good time, part of the fun is roughing it, that's the point so as long as you have food, protection from rain and cold and a few emergency supplies in case someone gets hurt that's all you need
S