OK, thanks Tommy, I didn't know that.
If fitting HID will indeed be different with wiring etc, as well as levelling and cleaning devices that must be fitted and work. Personally would think more hassle than it is worth. Also another (potential) stumbling block come NCT time... see below for copy/paste of latest NCT rules.
6. Where HID (High Intensity Discharge) bulbs are fitted switch on the headlights and check that cleaning devices
(where required) such as head lamp wipers or high pressure water jets are present and work. Check where
possible that the head lamp levelling device is working.
7. HID headlamps must be installed as a complete type-approved system. The system will include
automatic cleaning. HID headlamps can be recognised by:
a) the light having a bluish tinge,
b) the light taking a few seconds to reach full intensity,
c) the headlamp having an igniter module/inverter behind the headlamp,
d) the headlamp having ”DC” (for HID dipped beam), “DR” (for HID main beam), or “DCR” (if both are HID)
marked on the lens. This marking should be visible from the exterior of the vehicle but in some cases
the markings may be hidden under the bonnet and this may need opening to obtain a clear view.