In the movie, it's supposed to be odd. Gambon is playing an American general who after spending his childhood in English military schools never quite lost the British accent he picked up.

Anyone who saw his LBJ know that Gambon could nail an accent, it's the Irish in him.

He makes a great joke about it where he got de-elocution lessons so he could sound more like a US General.

I think it’s vaguely South African by way of the Lawrence of Arabia British Raj. That was another prescient part of the movie: as apartheid ended and the cold war faded there were going to be a lot of sociopathic white guys from former colonial states needing to make a living doing mercenary shit.