you dont need anything in or out, there is no official paperwork for a sub 12 rifle. However, I have a declaration from Walther, and an old one from the BFTA that says my gun is a sub 12 rifle... it looks vaguely official. But to be honest, they've only ever opened the cases once, and just given a look as if to say "hmm, i'm sure it's ok, cos i don't have a bloody clue what i am looking at"
The customs guy at Gatwick we had on the way back from Hungary was a shooter, so he was fine... he logged the serials and that was that. Rest of the time it's been an xray on the way out, and they don't know what they're looking at beyond making sure they don't see what they don't want to see going on board.