Accent wise as far as i'm aware has evolved to what it is today from the civil war period and the mixing of the settlements, however you'd be right in the words used in speech as American English retained quite a few words which were fazed out in British English

i'm no expert, so i could be very wrong, but as much as the standardised British English evolves quicky in term of slang the accent doesn't change a great deal, alot of our accents are ancient and have existed for a very long time in a similar form, so in all likelihood the accent at the time was probably alot closer to english than the current American English accent.

As i said this aint going to happen it was just a thought

This game is made by the French, what do you expect?

French Canadians...we actually like the British. 

Oh right, didn't know it was being made by Ubisoft Montreal.

Holy balls!

You're saying that you like the modern tie-in?

Not saying it was the same accent or even very close, I'm just saying it is closer than modern British English.
I'm studying English and I know this topic was part in one of my linguistic classes (I've learned the same in school already). Not only words (dialect), but also in pronounciation (accent).
For example the majority of the people in North America and Australia have a rhotic accent, whereas the majority of the British people have a non-rhotic accent, but they had a rhotic accent a few hundred years ago.

Would it not be knda cool if they threw a loop with this game and you could choose to be an Assassin working for either side? Not at the same time of course. 2 different paths through the game. Not unlike Folklore. Of course 99.9% of people won't have a clue about what I mean when I mention Folklore, but for those who do.

I have no problem killing Brits or Americans, they were both dirty slavers back at that time.

Why do you think that? I that it was stupid. It kept pausing the game for unnecessary reasons.

but that's just it, the modern stuff IS the story.  yeah, the ancient stuff is more visually appealing, but it's the plot that spans across time and space that makes it such a deep and engrossing story.  Ezio's story is a great plot in and of itself, but the fact that he's tied to Altair, this new hero, and dozens of others throughout history thanks to Desmond is what gives this game such a wide-ranging, poignant plot.