Ah, that's unfortunate you can't travel. I don't suppose you are anywhere near Camden? The Beaumont hospital in Camden is fantastic for rabbits. Yes, you are right, males can be neutered as soon as testicles descend, normally around 10-12 weeks. I certainly recommend that at least for the...
Is your vet on the RWAF vet list? https://rabbitwelfare.co.uk/recommended-rabbit-friendly-vets/ Rabbit vets in the UK do not recommend any preventative treatment for fleas or worms. Not a good idea to apply unnecessary chemicals or give unnecessary medication.
Not eaten in 3 or 4 days?! Poor bunny. He must have eaten or he would be in a terrible state. Has he eaten anything since he's been with you? He's very cute. Crossbreed. Thank you for giving him a good home.
Could be gut slowdown, possibly from ingesting too much hair. Could be stress, including stress from pain or illness. Could be not enough drinking. Could be not enough hay eating. Etc. What's his/her diet?
I would remove the paper. Ingestion of non-food stuff is never great. Try grass chew mats. Does she eat her body size in hay every day? Sometimes they get interested in chewing paper or cardboard when their gut is slowing down from not eating enough fibre for example.
Did you write another post about taking your rabbit outdoors yesterday? If so, the stress of that could easily have caused this. Take him to the rabbit vet ASAP, not eating is a serious emergency.
Whatever you do please don't breed, why bring more rabbits into the world where there is no suitable vet care... Besides, it is false that having a litter would reduce her chances of cancer.
I never use Pets at Home, a lot of us over here boycott shops that sell rabbits. I get everything online. If you are here for long enough, have a look at Twig & Nibble, Hop to Forage, Viovet, Hay Experts, Hoppy Pets,...
Wow, from not being sure about getting another bunny, you ended up with two :) Well done for not leaving the mate there, assuming they really are bonded. I strongly recommend having them neutered ASAP. They may get along now but that's not guaranteed to last. Also, obviously you need to make...