Stolen from another forum.

The upgrading of existing halogen headlamps with HID bulbs has been a MOT fail for a few years now. However upgrading with LED bulbs was OK.
From 11 January 2021 the MOT rules have been updated to make the fitment of LED bulbs ( in existing halogen units) a fail.

From the revised requirements. .

4. Lamps, reflectors and electrical equipment

4.1.4 Headlamps - Compliance with requirements - Changed sentence from '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 '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.'




Link to Govt website:
https://www.gov.uk/guidance/mot-insp...#section-4-1-4