Andy,
If you're just looking at PPL, a Class Two medical will see you through it. You can get it done locally; a list of pracitioners is on the CAA website. I don't know how much it costs, think it's around the hundred quid mark.
As far as I'm aware, to solo in US airspace, you only need an FAA Class Three, which will set you back about USD 75, and should be arrangeable through any flight school over there.
If you've higher ambitions, the initial Class One can only be done at Gatwick, and will set you back 400 quid. Renewals can be done closer to home, and cost about GBP 150, as far as I know.
As for medical fitness, go easy on the booze and cigs [looks at ashtray and empty bottle, notes a lifestyle change would be good]. If your blood pressure's high, kick off an exercise plan. But for most folks, the biggest problem can be eyesight, which is a bit harder to work on. Er, do you like carrots?
Good luck.