Googled...
"Under British Road Vehicle Lighting Regulations, LEDs are not mentioned, because they were not invented, when the legislation was penned in 1989, meaning that they are not permitted at all..."
It's not the "brightness" that's the issue, it's the position of the "filament" in relation to the lens.
The lens is designed to focus light emitted and a lens designed for a halogen filament allows "scatter" with other types of bulb. It's the dip beam that has the problem with "scatter".
Nevertheless, the design of Supra headlamps includes a "projector" lens for the dip beam and I'm pretty sure that it could be reclassified to be legal, it's the non projector headlamps that are the problem.