i found this website called the effing pot, with all kinds of british slang. here are my favorites:
knuckle sandwich – someone’s about to punch you (in tha FACE!)
nark – someone who is in a bad mood, or being grumpy
taking the biscuit – sort of like our ‘takes the cake’
donkey’s years – you would say this to someone like, ‘i haven’t seen you in ages’…just replace ‘ages’ with ‘donkey’s years’.