Ah..... In Q1 thread 13, you asked '...are silencers/supressors controlled items in the UK. Mmmmmmmm. Silencers per-se are outlawed. End of story. But supressors or what we now call 'moderators' are permitted subject to blah blah blah. Because by definition, while they 'diminish' the noise they do not silence it. This is the reason why the readily available (but NOT so readily available now.....) .22" P-H sound moderator is described as a moderator, it is not a silencer because the internal volume doesn't equate to 20 or so volumes of the barrel. Please don't quote this 3 liner as gospel though.
As for the moderators being proofed because they form part of the barrel and........ Nope, I don't understand it either. Nor, clearly, does the proof house understand the physics of what they have effectively passed into their monopolistic rules