you can do fairly much anything if an Dept Transport authorised engineer will sign it off.
Ring Transport they'll be able to give you the details of a workshop/s in your area that are authorised to issue 'LK1' mod plate.
Speak to these guys and they'll be able to tell you what you need to do to fit the new seats.
I had to get the seat rails supplied with my race seats modified to fit my car. The guys fitted a LK1 mod plate, it's all legal