Hi Karen,
I would still recommend your local rescue centres as your best bet. We rescued a dog before Christmas and they only asked for a £60 donation. I know cats are less than that though so you'd probably be looking at about £40 which is obviously a lot less than the pure breeds. Also the dog we rescued is a pure bred golden retriever, we couldn't believe our eyes when we saw him, which goes to show that you never know what you might find at the centres.
I find it useful to 'phone round vets to get an idea of prices, they can vary drastically so don't just assume they will all charge the same.
You might like to check out sainsbury's or m&s for insurance (M&S are slightly better IMO as they pay about £7000 per year whereas sainsbury's pay about £6000 per condition which might run out quickly depending on what it is!!)
Hope this helps and remember to post pics of your new kitten/cat when you bring him home!!