It's worth noting the the legislation says" rifled gun". That does not differentiate between rifles and pistols.
If the GTA wants to ignore all the advice for operation of this legislation issued by the Home Office and agreed by the CPS and ACPO they will also get pistols banned, although they apparently have no qualms about their members selling them.
The GTA would be better occupied by finding put why their members are not competitive compared to the E.U. dealers. Someone somewhere is pocketing the extra cash the U.K. dealer charge.