IANAL, but that gov.uk link (here: https://www.gov.uk/government/publications/aftermarket-hid-headlamps/aftermarket-hid-headlamps ) has some interesting wording.
First it says:
In the Department for Transport’s (DfT) view it is not legal to sell or use after market HID lighting kits, for converting conventional Halogen headlamps to HID Xenon. If a customer wants to convert his vehicle to Xenon HID he must purchase completely new Xenon HID headlamps.
Clear so far... Note also it is only an opinion of the department, and not legally binding.
It then says:
The Road Vehicle Lighting Regulations 1989 regulate the situation in the UK. Under these Regulations, HID/Gas Discharge/Xenon headlamps are not mentioned and therefore they are not permitted according to the strict letter of the law.
However new vehicles have HID headlamps. This is because they comply with European type approval regulations.
So under UK law, HIDs of any source are not legal. Also clear.
Under EU Type Approval (which the UK must accept), they are legal, so we defer to EU law:
For the aftermarket, a used vehicle cannot obtain type approval because this only applies to new vehicles. However DfT does not think it reasonable simply to ban HID in the aftermarket. Instead the Department makes analogies with new vehicles. It seems reasonable to require HID in the aftermarket to meet the same safety standards as those for new vehicles. The same level of safety should apply.
So it's illegal under UK Law, legal under EU Type Approval for new vehicles only, but then the Government choose to ignore UK law, and choose to bend EU law, and so make the sale and fitting of HIDs legal, and should (not MUST) comply with all the safety standards of new cars, *****which they just said can't happen under EU Type Approval*****.
So they actually say it is legal to fit them, because they also state it is not possible to get type approval.
Test case required, me thinks.