link not working for me, but it says...
4.1.4 Headlamps - Compliance with requirements - Sentence changed from 'Some motorcycles may be fitted with high intensity discharge (HID) headlamps. Existing halogen headlamp units should not be converted to be used with HID bulbs. If such a conversion has been done, you must fail the headlamp.' to 'Some motorcycles may be fitted with high intensity discharge (HID) or light emitting diode (LED) headlamps. Existing halogen headlamp units should not be converted to be used with HID or LED bulbs. If such a conversion has been done, you must fail the headlamp.'
Speaking to a mate who's a MOT tester, basically the bulb has to match the reflector so you get the right pattern...