Yup, it's an ass
but we have to work within it all the same.
It is easily open to strict interpretation and if your Police Force test with a certain pellet and find it over 12ft/lb they can royally screw your nuts to a cell.
Doesn't mean it will happen but....

There's been a huge amount of debate on here and the consensus is always to tune to well below for safetys sake.
Regarding all guns being CAPABLE, that's why the Home Office are currently pressuring manufacturers to make it impossible for us mere users to tweak our guns.
If the Police can prove that you tampered with the gun then you'll be prosecuted, and you can guarantee that really means that you have to prove you didn't.

My advise is to find the best performing pellet (highest ft/lb) and tune the gun to 11.5 max (remembering power curve for PCPs). Performance wise there's little difference between 10 - 12 ft/lb for hunting at air rifle ranges (sub 50 Yds) anyway. Anything else and you're risking your gun/liberty.