A is still a in both American English an British English. You can't just say that if everybody had the same pronunciations it would always sound the same. Everybody can play baseball the same way, but it doesn't mean they'll all win or all lose. It's still the vowel A. If we were to take a Southerner from America and a Bloke from Manchester and put them side by side and told them to say just the letter "a," it'd sound different, but it'd still be just an "eh" and an "ee."
If different accents have different vowels, then, it would stand that every human on the planet has different vowels, because Joe Schmo's voice is recognizable from Moe Schmo. If Joe's voice is different from Moe's, and thus is a different accent. Just because we associate accents with nationalities doesn't mean they have to be. In fact, here's the definition:
Accent: a distinctive manner of expression: as
a : an individual's distinctive or characteristic inflection, tone, or choice of words —usually used in plural
b : a way of speaking typical of a particular group of people and especially of the natives or residents of a region
This is from Miriam Webster, because I wouldn't want to offend your highness with some online dictionary. Why are you so eager to pick fights? Did you get kicked from the debate club because you said something mean about somebody's mum? OK, spelling is useless, fine. Sure, maybe you need to draw a line when one sound is this and when another is that, but who's to decide where that line is? Why is a British accent comprised of different vowels from American? There's only so many sounds that a human mouth can make, don't try to make it more complicated by cutting it into even more pieces.
And when will you get it through your thick skull that it's a game about dwarves, and that this is a language that doesn't ever need to be spoken? Why do we even need to talk about pronunciation when we could have instead used a bunch of dots and dashes in place of letters? Are you gonna say now that a German straight key makes different dits n' dahs from an American one because it's a different pitch? Are you gonna say that a B # M is a different chord than a C M just because it uses a different name? How far do you take this? Am I a different species from a red haired person because I have black hair?
By just saying things are different because of minute changes is illogical and rather silly, you have to give room for the human to interpret. A B#M is the same chord as a CM, there's no way for you to say otherwise. A German straight key makes the same dots and dashes as an American one, and a British accent uses the same vowels as its American counterpart.
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And then, with all your naval gazing, you think that because I don't see the imaginary line in the sand which you've drawn, I'm an idiot and ignorant of language? I'm not even going to waste my time in saying that I'm sorry that you're such a small minded person. There's just no point. Even if you win, what will come of it? You go back to your mother's house and try to look for fights, and I go drive MY Mercedes to MY house, and then I get to go travel around the world for work. Do you think I really care a bit that you think I don't know about language, which by the way is blatantly untrue? Sure you can say I may not "know" linguistics, but to say I don't know language is almost insulting.