In the future the Brits drive the Americans out of all the big cities making the accents of the upper class all British and the lower class American. But the idea of Briton driving American out of anywhere is preposterous (no offense all you Brits).
Or maybe it's because they simply wanted a contrast between the way the protagonist talked and the way everyone else talked.

Don't you know? If your Evil in video games or the movies, there are only 3 types of accents: British, German, or nondescript middle eastern. SURE you have the occasional "rule breaker" But honestly, we all know those are the only *real* evil people in the world, heh.
(if you don't see this as humor, I am being funny so don't get your knickers in a wad :P..well I should say *I* think I'm funny...your mileage may vary..or kilometerage..whatever...)

My guess is that they thought that the one american fellow would sound funnier. And let's face it; for the most part, he does.

The true inhabitants of the city have one accent, the main character (brought up in the gap) has another, and Eduardo (who comes from a rival city, Hobart) has a third. That makes perfect sense.
Which modern-day countries those accents are from is irrelevant; all that matters is that they do contrast. My question (and an easy one to answer, but I don't have time right now) is what accent do Foster's adoptive tribe in the gap have? I would expect it to match Foster's own.

That actually makes alot of sense.

There are six speaking parts in the intro.
Robert: American, as noted.
Old Man: I don't know why, but I can't place this. It's definitely not the same as Robert's, though.
Reich: American. And he's from the city. (And he also sounds a little like Dr. Girlfriend.)
Unnamed Gap Person: This one speaks up after Robert is found as a child, and has an Australian accent.
Unnamed City Soldier: American.
Unnamed City Pilot: American.
So... um...

The game being in the future, an alternate reality, in addition to the development company's natural location, I am sure influenced your, above, accents. I would best imagine the Australian-British-American connection as a creative decision--artistic direction with the artistic license that comes with being an artist; video games are art, as are films, fine art pieces, photos, sculptures, buildings, songs, poems, novels, tapestries, and any thing else that requires creativity and passion. I hope I helped.
For more of me, look my screen name up on your favourite search engine.

well there are some other bits, in the factory, after breaking the machinery if you look at it foster will say "busted" in voice and "knackered" in text

Well, I live in Australia, and not many people have an Australian accent nowadays. I picked up an American accent from watching too much Sesame Street when I was younger, and this applies for a lot of people I know. But I don't know anyone with a British accent here.