I think language changes quite rapidly. I am British but have not lived in Britain since 1991, so I can spot current British usages that (I believe) were NOT generally current prior to 1991. Here are some random examples;
"Can't be arsed" (don't know when it was coined, but it is new to me.)
"Hiya" (Australian in origin, I think?)
"Uni" (for university. Again, Australian, I think.)
"Bin it" (verb for throwing something away.)
"Wheelybin" (an obvious coinage - of course years ago, bins did not usually have wheels.)
"Brilliant" (as an exclamation, as mentioned in previous posts.)
Now, all of the above colloquialisms and new coinages are clearly NOT American. So I think British English is very dynamic and open to new usages ... American English is only one of the sources.