There are many pitfalls in the legislation but this is more than well known.
The correct action would be to get what is causing the problem altered. Currently this is the wording on the Statutory Instrument and the lack of a standard test procedure.
Both of the above can be fixed without any increase/decrease on the power levels which would leave the individual airgunner more than able to ensure he stays the right side of the legislation.
The fitting of AT, putting the holder of the rifle in an untenable position of being responsible and liable but not being in a position to do anything about it is not the way to do it.
It's the only legislation I know of where the individual subject to it is prevented from complying with it.