Vets fees
All pets will need to go to the vets at some point; hopefully they will just be for annual check ups and nothing serious, but even those check ups will cost you. As well as these check ups some pets need injections as well, these are generally against flees etc. Some vets also advise getting them injected against serious diseases they could catch, all in all vet fees set you back a fair amount.
Annual boosters for a dog tend to be around £35, this will obviously vary for each veterinary group you go with. An annual booster for a cat can cost up to £38 but again this can vary, however you should also have pet insurance which can cover these fees so you may find that these injections won't actually cost you a penny, which is another good idea about getting your pet insured.
It is now illegal to have a dog and not have them micro-chipped, if your dog is found to have not been chipped then you will receive a letter warning you if they are found again without a chip then you can get fined up to £70! Micro-chipping is about £30, though you can cut this cost in half with some pet insurance companies will offer to pay half of it themselves as part of their service.