However, the original post and subsequent answers from .277 appear to be asking a different question. I may be wrong, but he appears to be trying to buy a moderator from an RFD which he states he isn't going to use on a firearm. He's then asking why the RFD won't sell him a moderator without going down the official route.
I think that an RFD, sensibly, won't sell him a centrefire calibre moderator without an official acquisition transaction. Yes, in certain circumstances a moderator is just a baffled tube of metal, but any RFD is going to be mighty suspicious as to why anyone would want to simply possess a 300 quid baffled tube of metal in the first place. Folk muddy the waters by drawing a parallel with air rifle moderators, but times have changed, and so have air rifle moderators. Many modern air rifle moderators are fitted with plastic/polymer baffles and sound deadening material, so aren't suitable for propellant burning rifles anyway.
As a former cop, I'd want to know why he wants to buy it. As a former FEO I'd advise him to apply for a variation and buy one officially, and as a former RFD servant I wouldn't sell him one without an authority to acquire.