Hi Paul
I don't think it should be an issue with non-standard bulbs as it is the deflection they are looking for rather than the light output so LED bulbs should be okay as long as there is no chance of causing glare to other drivers. Main beam generally doesn't have any deflection and is not a controlled light source, unlike the dip beam. So only difference is the light output will be increased and maybe with a different tone (blue, white, etc).
I put a car through an MOT recently and it had LED sidelights and indicator bulbs and there was no issue.
Will let you know if I do find any conflicting information though.
Cheers, Trevor