The irony of the Bren DP saga (only real, MoD spec L54/5's of course) is that they were actually being sold as deactivated, off certificate during the 80's BEFORE there was even such a category as 'deactivated', for the princely sun of £57 or so. This was on the basis that they were not firearms as defined and as such, exempt from the act. Mind you, when you could buy the real McCoy as a single shot firearm or a smooth bored shotgun for the same, then quite understandably, Mr Pannell didn't sell many!
I saw one recently that had been purchased as an L54 DP Bren prior to the 'ban' and subsequently re-submitted for certification. Just the barrel welded in and it passed. But was there even a need for it to have been re-submitted on the basis that the law isn't retrospective?