EJ - the modern consumer has well known choices as to how to approach a major gun purchase. There are something over 50 gun making names that are capable of getting out an arguably best gun. Their methods are international and they approach runs from strictily in-house to completely international. There is some variation in pricing, but a bespoke best gun costs a huge amount of money. If you want to be sure that you are getting a London made best gun, go to Purdey's/H&H/Boss. If you will accept a "Birmingham"/trade best gun, then Evans, Westley Richards, W. Richards, Greener, AG&L, Hellis, Bozard, AA Brown, McKay Brown, Boswell, etc. If you have some flexibility in style (but not in mechanical quality), try one of the several German, Italian, Belgian, French, or Austrian makers.
Are you thinking that London is the only place that has the skills necessary to get out a best gun? Or, are you thinking that the London style is still truly unique to London?