I think that most training is in-house and, due to the demise of the British gun trade, opportunities are few and far between.
Perhaps an option is to rattle up a stunning CV and send it to the few remaining british gunmakers.
Also think about the different trades in traditional gunsmithing, although many are now "jacks of all trades" the specialist powder burner makers still have individual barrel makers, actioners, engravers, stockers, finishers etc.

I wish you the very best of luck and hope that your search is successful.

ATB

Richard