Read the spec on the power supply (it'll be written on it somewhere) Check the INPUT settings - usually they can cope with 110 to 240v. If it just says 240v then you should get a new US supply. The current won't be a problem unless it's also a problem in the UK!
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed. Here's an example...
“I think it’s wrong that only one company makes the game Monopoly.” – Steven Wright