They interviewed a policeman who said how easy it is to cut a .22 rimfire rifle cartridge, (the pistol cartridge is illegal), down to fit the chambers of a Brocock air cartridge revolver. About how it would be necessary to put the round into a specially designed receiver tube, to take the force, keep the round central in the chamber etc. They didn't say anything about the difficulty in obtaining the ammunition, or the skill and facilities required to cut down the cartridges, reload them again, engineer receiver tubes and make any other necessary modifications to the revolver. I believe it is sometimes necessary to put in a new barrel. The funny thing is that after his explanation of how bullets for the gun have to be modified, along with the gun itself, the silly reporter just went out to the local sports shop and bought a Brocock air revolver and then said how awful it was that she had been abe to buy a gun capable of firing real bullets over the counter!
Reactivating replicas and deactivated firearms is more difficult. If the criminals can get illegal pistol ammunition, to fire, then for the same money they spent on a replica and its reactivation they could just as easily get an illegal firearm from the black market. In all variations the .22 rimfire remains a relatively low-powered and low-pressured round, so its not the kind of round a gangster would use if they had a choice. Also, what self-respecting Yardie is going to carry a converted .22 calibre air revolver? They'd far rather carry an Uzi, or a 9mm Beretta, or a .44 Magnum, or a Browning L9A1, or whatever.
In fact, reactivated or converted guns make up only about 10% of the weapons seized by the Police, and they are the least dangerous guns out there (from the victims point of view) - how many blow up in the perp's hand? The government has no way to stop the other 90%, so its trying to show how tough on crime it is by targeting one of the sources of the 10%. The problem is the criminals will still get replacement guns from somewhere and the government will just be bankrupting British airgun manufacturers and importers.