From 'Matters of Testing MOT Issue 53 April 2012'
Aftermarket motorcycle lamps
All manner of aftermarket lighting products are available to customise motorcycles. We look at the legality of some of these products as well as what happens if they are fitted at the time of MOT test.
LED lamps
An acceptable alternative to standard bulbs, these lamps tend to use multiple LEDs. The MOT requirement is that at least 50% of the LEDs must illuminate. If not, the lamp will fail for being ‘so damaged or deteriorated that its function is impaired’.
Combination rear lamps
Such lamps usually have a clear lens, which is illuminated red when the rear position lamp is on or the brake is applied. If a direction indicator is switched on, half of the lamp flashes amber, whilst the other half remains the red of the stop and/or rear lamp.
This type of lamp does not meet the regulatory requirements and is therefore illegal. If one is fitted to a machine presented for test, then it should be failed for the stop and position lamps being adversely affected by the operation of another lamp.
Flashing stop lamps
These incorporate a modulator which causes the stop lamp to flash rapidly for the first few seconds of brake application, which gradually slows until the light is permanently on. These lamps are also not permitted by the lighting regulations and will fail an MOT test for emitting other than a steady red light.
Flashing stop lamps which flash under heavy braking are permitted for passenger cars, but these are subject to strict type approval criteria in respect of their activation
HID headlamps
Due to the adverse affect on the beam pattern, it is not considered legal to convert conventional halogen headlamps to HID xenon using the existing lens and reflector. Fitment of complete HID xenon headlamps is permitted, although the lamp may still be required to have some form of self levelling system to be legal.
In either case, there are currently no Reasons for Rejection in respect of motorcycle HID headlamps, providing, of course, that the headlamp aim requirements are met.
I knew fitting HID conversions to cars was illegal - looks like it's ok for bikes though

'02 VTR1000-FY Yellow.
'12 Moto Guzzi Griso 8V SE Tenni.