Frome the GEM website:
"Not only is it an offence to supply car parts that purport to comply with certain regulations, yet do not, but also fitting and using them on the road is not permitted. However, it is possible that a car can pass an MoT Test with an LED conversion bulb fitted to an exterior lamp; this is because the MoT Test, generally, does not enforce Type Approval laws. Yet, you still risk being stopped by a police officer and prosecuted for driving an unroadworthy vehicle with them installed, as well as invalidating your insurance cover.
"New bulbs are one of the few aftermarket replacement parts that have to comply with compulsory technical standards. The UK’s Road Traffic Act (1988) cites that it is an offence to supply, fit or use vehicle parts that do not comply with the required Construction and Use stipulations. Selling an LED conversion bulb might incite the driver to commit the offence of driving an unroadworthy vehicle, leading to a risk of a prosecution under Road Vehicles Construction and Use Regulations (1986) and the Road Vehicle Lighting Regulations (1989). Not even UK companies that specialise in automotive lighting and retail LED bulbs could tell GEM why they might not be breaking these regulations."
If you modify your vehicle in a way that makes it unroadworthy, then your insurance will be invalidated.