I think in England it's £6-something, but in Wales we pay £3 per item. Regarding your prescriptions, you'll have to go to your local surgery to find out what the UK equivalent of your meds are. They may be the same brand name as in the US, but perhaps will be known as something different over here. Rather than bringing your US prescription, I think you'd be better off bring the actual packaging from your meds -- that way your UK GP can look at the ingredients, look it up in his book and find you the equivalent.
(If you're receiving benefits in the UK, I think prescriptions are free, but I don't actually know how that works.)