Hi,
I'm going to tell you the most noticeable differences I notice between British English (the only variety of BrE I know), and American English (the variety of AmE I hear most often).
CAR ------ American R's are always pronounced, British R's are not.
NO -------- British is /əʊ/, American is /oʊ/
NOT ------ British is /ɒ/, American is /ɑ/
NOT ------ British final T's are released, American final T's are not.
LAW ----- In BrE it's /o/, in AmE it's either /ɒ/ or /ɑ/
CLASS --- Some vowels that are /ɑ/ in BrE, in AmE are /æ/
BETTY --- In BrE there are no tapped T's, in AmE there are.
NOW ----- In BrE it's more like /ɑʊ/, in AmE it's more like /æʊ/
Those are the differences I notice. There are a lot of regional variations of course. It would be interesting to know if some American features are present regionally in some other places (for example tapped T's, unreleased final T's, etc.)