There is no doubt in my mind that the finest artisans of metal working in the guntrade were Birmingham based or trained.Richard Akehurst documents this in his book highlighting the fact that in tests carried out by the Birmingham Proof house 'Black Country' barrels were far superior to Belgiun.The first ever Iron bridge was made in Ironbridge in Shropshire 25 miles from Birmingham.Rapier steel from Toledo used Birmingham know how and currently Bohler of Austria have used Brummie input.Nice to know that in the '60's Britain supplied 36% of the World's Engineering technology now I believe it is down to 13%.
As regards London / Birmingham quality it is an easy explanation.London concentrated on marketing and sales techniques and Birmingham concentrated on manufacture.
H&H used Birmingham workshops and craftsmen,so did Churchill and Purdey.And surprise surprise they still do.But I bet they wouldn't like me to tell you who and where they are.
Suffice to say many 'Best' guns are made in Brum, proofed in London, finished in Brum, sold in London, used in the US.Everyone lived happily everafter.The End.