MoT rule on this have just changed apparently
https://www.gov.uk/guidance/mot-insp...#section-4-1-4
"Existing halogen headlamp units should not be converted to be used with high intensity discharge (HID) or light emitting diode (LED) bulbs. If such a conversion has been done, you must fail the headlamp."
Whether this makes any practical difference remains to be seen. As ever the only actual sin is getting caught.....
Nick