The bars are standard diameter so you could, at least in theory, fit other shifters. They are a bit narrow though, so a Raleigh Grifter twist shifter (the only SA gripshift I know) would leave no room for a normal grip.
The standard Brompton triggers work OK, they just look rubbish. I thought about replacing with a 1970s chromed SA trigger, but although it's an infinitely higher quality item, it wouldn't suit the bike.