An absolute must is insurance. I had a cat that unfortunately got cancer on his lymph nodes, and at the time when we had to put him to sleep, the running vet bill was £2000.
I always have pet insurance now, and the cat I have at the moment, has been diagnosed with auto-immune anemia. This is very rare in cats (just my luck!) and costs £50 every other month for blood tests and medication. There is no way I could afford it with out insurance. Watch the small print though, as you will find most will not cover dental, which shouldn't be a problem until the cat is older.
Insurance is about £7pm with More Than. It may be less with a kitten, but my cat has an ongoing condition hence its a little more.
Food wise, I feed my cat with Sainsbury's perfectly meaty/fishy and Encore dry mix. This works out to be around £20 - £25pm on food. I also give him coley and roast chicken a couple of times a week just to break up the boredom of the same food day in day out but I am just a big softy!
Flea/worming is around £25 (i think) and you will need that every three months
Cat litter is much of a muchness tbh. You can spend a couple of quid, or several, depending on how much you can cope with the smell! (dont use paper based tho, they are crap!)
Spaying/neutering varies depending on your vet. You can get it done for nothing or at a reduced price if you meet the criteria.
I would never be without my cats. I have had them since I was 8, and I am now 36! Toms seem to be more affectionate. Females can be a little offish in my experience, but she was lovely all the same. (she disappeared one night never to be seen .. I was devastated)
Good luck in your search