What´d be the main difference between such expressions? the former is used in AE and the Latter in BE?
A u-turn means that you turn and go the other way on the same road.
A detour means you take a roundabout route to get to your destination - a detour is what happens when roadworks are being carried out and traffic is diverted onto other roads.
They are not synonymous and both are used in BrE.