As neither an American or a British person, allow me to give an outside opinion.
I have seen people with British Accents ridiculed or straight out hated by American players on many occasions.
With a very few incidents of it happening the other way around, except in cases where it's not really the accent that's the problem, but rather the attitude of the player. With nationality simply being brought up for the purpose of mocking them, rather than being the reason behind it.
Not saying that British mocking Americans simply for being Americans hasn't ever happened, it's just that I see it happening a lot less often.
Due to time zones, it's somewhat difficult for me to get into matches with those of the British persuasion, so I generally end up playing with Americans or Australians (damn them).
The good thing about British people, is that they recognise my accent, but don't mock me for my nationality like a lot of Australian players (damn them) have.
The good thing about Americans, is that they generally don't have a clue what accent I have and spend valuable mocking time trying to figure out where exactly I'm from, before either giving up or settling on just calling me a stupid foreigner.
I'd much rather play with British players, they have much more awesome accents and I've heard enough angry, racist, homophobic and just plain old annoying 14 year old 'mericans screaming through a microphone to last me a lifetime now.
fun times.
