Maybe we should come up with what we think is best practice for keeping your gun legal.
Not all guns are at risk here, for example my TX runs at about 11.2 fpe, there is nothing the police could do to my TX to make it exceed 12fpe.
They could fit a stronger spring or add some piston weight to make it exceed 12, but if that were the criteria for being capable, every single spring gun available to purchase from every RFD would also be capable too.
I believe that puts me in a very good legal position that I would be confident to argue in court. The argument being that if I'm guilty of holding a Section 1 firearm, so are all the springers in all of the shops.
That's where AT might be a good thing for airgunners. All you need to do is buy a gun with AT, before you take delivery insist that it's tested with a selection of pellets to ensure it's running at a safe margin ( around 11fpe ) and keep a regular check on the power.
You then have the same argument in court - if mine is 'capable' so is every gun in the shops today.