They say that the USA and the UK are "two nations divided by a common language". I have to say that having met a huge number of people from America, I have never had any problems understanding them - many people have, however, had a great deal of problems understanding me! I know that I need to be careful sometimes - in my days of smoking, my statement that I was going to light a fag often got me some very strange looks. Admittedly not quite such strange looks as when I went to a tobacconist in France, thinking that my request for cigarettes 'sans preservatifs' would result in cigarettes without any sort of preservatives in - sadly I was actually asking for cigarettes 'without condoms'!
Some things are basic differences every British person knows. 'Pants' to you are 'trousers' to us. To a British person, 'pants' is a word only used to describe underpants - and also a slang term for something being useless.
'Randy' is a first name in America. If that happens to be your name, please don't come over to Britain and introduce yourself to someone with the statement "Hi, I'm Randy." Unfortunately you will be introducing yourself with the statement "Hi, I'm horny." Similarly, a 'rubber' is only a pencil eraser. Don't panic if it gets mentioned.
Jelly and jam can get a bit confusing. When I was told about 'peanut butter and jelly sandwiches' I understood that to be what you would call 'peanut butter and jello.' Didn't sound too nice to me! To me, what you call jelly, I call jam.
There are some other British words it would be useful to know:
arse: ass (but more rude)
chat up: pick up (a boy/girl)
crap: useless (and a bit rude - not too bad, but don't use it in polite company!)
dodgy: can't be trusted
fit: as well as being a term used for being in physically good shape, it can also be used to describe someone as attractive.
kip: a short sleep
knickers: ladies underpants
knock up: to get someone pregnant (certainly worth knowing!!)
lift: what you call an elevator
mate: friend (nothing more, don't panic!)
sacked: fired (from a job)
shag: yes, it does mean sex
twit: how to call someone an idiot without really being rude!
wonky: shaky or unstable
yonks: ages, a long time
I could start on 'cockney rhyming slang', which is a traditional London thing, where people say words which rhyme with the one they really mean. For example, 'apples and pears' means 'stairs' (ie staircase) and 'dog and bone' means telephone. But to be honest, I think that needs a whole new article of explanation!
I hope that my knowledge of American-English has been good enough fo me to convey things properly! Any questions, please feel free to get hold of me. By the way, that means contact me, not grab me by the shoulders or anything like that! :-)