People in the U.S. say the word 'brilliant', but not the word 'spiffing'. I had no idea 'brilliant' was a solely British word.
That said, I assume you guys in Britain get all the crap movies that come spewing out of Hollywood each year? I assume you guys get some of our cruddy tv shows? How about our cruddy reality tv shows?
People are influenced by what they see on television. A lot of people watch a lot of television, and if you watch it for hours every day and hear the same things, you are likely to pick things up. Kind of like how I started saying "aye" at the end of sentances when I was living in Canada.
I hardly think that the sayings and words people use in Britain mean that Britain is becoming just like the U.S. I doubt that Britain is not influenced by the U.S., but the U.S. being such a big country with such a large influence, there is hardly any place in the world not influenced by us. If you want to complain, complain about McDonalds, Dominos Pizza, Papa Johns, and various other chains that have spread their way across your country. Complain about the fact that American movies and TV shows are shown on your televisions (if they are that is).
Words don't mean much, in my opinion.
But maybe I'm just saying that because I live in NYC, and it seems I am introduced to new language every day - whether from another language entirely, 'ebonics' (e.g. non-English English made up by 'ghetto' people), or just...English words that I am not familiar with
Nothing wrong with it.
*edit* P.S. Halloween is fun! Maybe people just enjoy dressing up and getting/giving out free candy...