Warranty in the UK is discretional on camera bodies if camera bought outside of EU. You'd have to declare it on entry to UK leading to import duty (c. 5% I think) and VAT (17.5%)
So, $5000=£3100
£3100+5%=£3255
£3255+17.5%=£3825
So, is a warranty worth £200 on a new model? To me, yes.
Also, if you buy in a store in the US you will pay State sales tax in addition to the price (e.g. if you buy from B&H in NYC you will be charged 8.875% on top of the sales fee - enough to make the US price more expensive than the UK)