It's deactivated. The process is quite comprehensive so while it looks
the part, cycles and is intact enough that it can be taken to bits to see how it works, it'll never fire again. The process involves milling away at least half the bolt face at an angle of at least 45⁰ and weakening/destroying/removing all other load-bearing surfaces, in its case the locking block in the upper receiver (gone) and the gas piston (unscrewed from bolt carrier and discarded, and the part it screws into having slot cut along its length). The barrel has a slot cut in its underside for about half its length, has the bore drilled out and a steel rod inserted and welded in place, has a hardened steel pin blocking the entrance of the chamber and is then welded to the receiver to stop it being removed. Which I think
is everything; the bolt still has the firing pin (albeit ground down with the bolt face), the trigger mechanism still functions as it did originally and so on. The BAR Model D and MG42/53 have undergone much the same procedure, in the case of the latter the rollers have been removed as the load-bearing components, barrel and bolt treated the same way as with the Bren.
The regulations have become significantly more stringent recently, mainly due to weak politicians and media pressure where it's wrongly claimed that the above can be reactivated in a back-street workshop in under half an hour, which is obviously nonsense unless they mean with a new bolt, receiver and barrel, in other words essentially an entirely new gun. Rather worryingly, this take was claimed to be true by a police "expert": not sure if he wasn't as expert as he claimed or if he was just lying. So the bolt and carrier have to be entirely destroyed, a facsimile welded in place and the trigger mechanism also filled with weld. It seems absolutely pointless except as an act of vandalism which will prevent people interested in the history of small arms from studying them. Existing deactivations are unaffected due to laws usually not being retrospective but they can't be "transferred" (i.e. sold, inherited or gifted) without being re-deactivated to the new specification.
My own opinions about the legal ownership of live weapons are rather ambivalent and I'm unable to reach consensus even with myself. I think the laws are much too draconian; on the other hand an e.g. .303 Bren gun is an exceedingly powerful military weapon and I think I would be a bit uncomfortable with the idea of it existing outside of a military armoury.
That said, I'm reminded of when I was younger and daft(er) and spent a while in the Army being made to do the graveyard shift patrol of this really, really
creepy old Napoleonic fort. They have me a sub-machine-gun and no ammunition. I would sort of try to look as menacing as a scrawny teenager with an unloaded gun could manage as I walked though deserted moonlit courtyards and past various archways with impenetrably black inky shadows. No ghosts nor vampires accosted me so it seemed to have worked.
Edit: here's a photo I found of the Bren's innards (well, actually a previously owned one, but they're much the same) so you can hopefully see what I'm on about. I'd forgotten about the hammer/unlocking cam which also has part of its face ground away, the bit that would strike the firing pin.