Since this was a government decision, it was burried in the cabinet papers and treated as secret till a few years ago. I have not seen the papers myself and I wouldn't go to court with this but I have been told that the move was actually instigated by the British Air Gun manufacturers. Back in the 60s the most powerful British Air Guns were producing around 10~11 ft.lbs. Then the German and the American guns appeared that were considerably more powerful and the British instead of upping their game decided to become protectionists and approached the government to bring in the sub 12 ft.lbs law.
At the same time an idiot who wasn't firing on all cylinders in his head, decided to take pot shots at the passing traffic with his air rifle and some news paper took it upon itself to protect the British public from this new weapon of mass destruction and the rest as they say is history.
That is what I have been told so it may not be kosher but it does make sense, knowing how things work here. If you want an air rifle that is even 0.5 ft.lbs above the 12 ft.lbs limit then you have to apply for an FAC ( fire arm certificate ) as these are classed as fire arms and getting caught in possession of one without an FAC is a very serious criminal offence with a possible lengthy jail term at Her Majesty's Pleasure.
Now the problem we have is not so much the 12 ft.lbs limit as people usually do OK with this ( they have to ) if the ranges are kept short. The problem is with the wording of the law which says, that an air rifle is considered a Fire Arm if it has the capablity to exceed 12 ft.lbs, or something very similar to this. This is a catch all law as any sub 12 air rifle is " capable " of exceeding this limit. So when the cops get hold of your gun they don't test it with the pellet that you were using but it is tested with a variety of pellets that you may not even had heard of to see if the rifle exceeds 12 ft.lbs. If so then you are in possession of a fire arm and liable for prosecution.
Therefore in order to stay on the right side of this stupid law we tune our guns to about 11 ft.lbs and some as low as 10.5. A few years ago the manufacturers decided to put AT ( anti tamper ) on the guns in order to appear to" self police " the air rifle community and any imported gun has " voluntary AT " installed.
Getting an FAC is also very difficult as not withstanding the gun security issues, special safes, alarms, background checks, health checks etc, you also need to give good reson why you'd want a 16 ft.lbs air rifle for example and where you would intend to use it. It is not acceptable to say that you just want one. I think that eventually they want to get rid of wide ownership of air rifles and guns in general but I am sure that the gentry and the rich will still be able to keep their guns but the ordinary public .!
Regards,
A.G