Would an american or whatever headlight have the appropriate markings on to pass a UK MOT?

Paul.

The lenses used in the ducati designs headlamp are made by Hella and carry E markings so are legal for road use.
As for an American manufactured one, good question but I've never known an MOT station check markings, only beam patterns.
Do Hardley Dangerous and Buell headlamps have Emarks????