Okay, you're not actually from London and your British accent is rubbish at best, but with this simple guide you'll be grabbing pints with the locals in no bloody time at all! A visual comparison of British phrases versus their American counterpart.
BRITISH vs. AMERICAN English vocabulary. a few others: US -- aubergine (UK), eggplant (US). courgette, zucchini. pants, underwear. trousers, pants. chat sb up (in a bar), hit on sb. play up, act up. be in hospital, be in the hospital. . . . AND many more.
Haha, I'm seriously thinking this sounds more Downton Abbey than modern British! What I learned from Doctor Who and Sherlock was that some British people actually do use the same slang as Americans... :'(