Speak to your local CAA regional office and get their advice. However, i beleive that it will be a major mod.
Ask the STC holder if they have sold any kits to another operator in Europe. If the answer is YES you should be able to check with that local authority that the STC is approved. Them just fit it.
The LBA have a very good section on their website listing all the STC's they have approved.
Best of luck
ACX