I understand your concerns, but it really is the first use of the vehicle after 1986, so if it had halogen lights fitted in 1990 and then the halogens are replaced with LED in 2015, that's all legal.
Basically, The Road Vehicles Lighting Regulations 1989, do not mention LED or HID lighting, but they do say, at Reg 4 (and I paraphrase it a little for the sale of simple English) "Where a motor vehicle first used on or after 1st April 1986 or any trailer manufactured on or after 1st October 1985 is equipped with any lamp it must be marked with the appropriate Standard CE etc marking'...
So anything post 1986 can't just change to LED lamps.
But our cars are pre '86 so those Regs don't apply. Therefore we can fit LED etc.
However, all vehicles must abide by the same Regs, Reg 27....which again to summarise it into plain English, says that all Headlights, Fog Lights etc are prohibited from use "..so as to cause undue dazzle or discomfort to other persons using the road."
HID lamps require specific HID reflectors because they are really technical creations, but LEDs are made to replace the standard Halogen and if correctly fitted are no issue.
