First of all; big necropost. ?
Secondly; UK law doesn't recognise DMRs or anything like that regarding Airsoft.
You have the legal limit for single action and semi automatic at 2.5J iirc, but for safety's sake, sites will obviously set it at a lower level, which for DMRs is generally anything from 1.48 to 1.88J (Approx 400 to 450fps when measured on a 0.20), or 2.3J for single action rifles, but that's for safety & insurance.