after market HID kits illegal but openly for sale... mmm... do they have the phrase "for show use only".. most dont.
they are not illegal. for those who want to go into very fine details about law may start quoting CE marks and IVAs as kit cars have to complete..
hang on, that would mean everytime you alter from the manufacturers exact spec would mean a car is illegal... different tyres, alloys, suspension, remaps, exhausts, body kits, bumpers.. blah blah blah.. wipers!!!?!!!! bulbs.... and so on..
The simply fact is, if its deamed safe, (there are no dangerous dangling wires) and it meets the MOT testing procedure, - light output alignment etc then you will be fine.
This is where the issue is misunderstood, - correct alignment. there are many people out there who have retro fitted HIDs incorrectly, with wrong bulbs or headlight types and so drive round blinding other drivers.. and its not the HIDs thats the issue so much as simply alignment. so yes you may get stopped in such a case and might get a vehicle defect rectification ticket (or whatever its called) which would mean going to an MOT place to have car tested for the defect to be corrected/tested.