Yes, just had it confirmed, deffo illegal.
Mechanic at local test station reckons upgrading to 100w bulbs could do the trick, he's going to check with sparky tomorrow.
Might be a much cheaper option, though I haven't looked at the "100 %" brighter ones yet.
There is also a HIR type bulb which is halogen infra red. This is supposed to be loads brighter without the ballast needed for HID or the legality problem. More sniffing on the interweb needed!
Interestingly, MOT man said that the problem with HID fitted into halogen reflector is they are not designed the same as a HID lamp and so would possibly not give correct light pattern and therefore would be a fail. However, if the beam pattern was ok and they were adjusted correctly (spot on), he would pass.
He who dares??