You may think that the British and the Americans speak the same tongue, but as George Bernard Shaw once said, we are "two countries divided by a common language."
For example, a "truck" in American is a "lorry" in British; Yanks say "dumbstruck" while Brits say "gobsmacked"; in England you "knock someone up" to call them from sleep...
Translate American English to British English
Translate British English to American English
The British Tourist Authority also has its own Talk Like a Brit Webpage!