As some have said above at best it would be a few years before you were allowed to work on a gun subject to being competent as well.
Theres a few decent gunsmiths and gunmakers around the Hertfordshire region. Atkin Grant & Lang may be worth a try, i know at least some parts are/will be hand made, they are making guns again. Jason is quite good but dont push him it will backfire. I reckon almost on a weekly basis these types of places are bombarded with such a request and subsequently go straight in the bin. Quite frankly most places are now so small they cannot afford to take anyone on. Add to this the lost time in training someone up based over many years and the amount of lost production probably 60% in first year, you get the picture.
There is a severe shortage of semi skilled and skilled labour up Scotland, three years ago i could have got you something in Glasgow with a highly regarded thriving manufacturer but had to resort to other means.
H&H only employ engineers, the skilled employees were all sacked over 15 years ago like 98% of the rest of the trade, who either set up on their own, retired, emigrated or died. Purdey still have barrel makers.
Wish i could get out of it.