Think LED's are Legal it was the xenon or something headlight conversion that was illegal because they would dazzle oncoming cars something to do with the reflector. this is remembered from deep in my tiny brain and could be a load of rubbish not sure lol
All a bit of a grey area really. HID's were meant to be fitted with washers, as the lights could dazzle off refractions on dirty lenses. Also they should be fitted with projector lamps( the glass convex in front of the bulb).
Most cars getting the upgrade to either HID or LED won't have projector lamps so run the risk of a poor beam pattern outcome.
I did a hi-low conversion to HID on a mk2 mondeo and it would pass MOT's fine as I don't think there is much in there outside of beam height to test.