As noted, the Brits made guns that can be compared directly to US guns in terms of intended function - wildfowlers and gamekeeper guns being the most obvious examples. I would be interested to hear a gunsmith compare the form, fit and functionality of a provincially made, A&D boxlock or a Bland wildfowl gun against a contemporaneous Parker PH or VH.

Might make for an interesting DGJ article?

A quick review of GI shows high-end Parkers with selling prices ($30-50k) comparable to what a vintage Purdey or H&H might get. There may be some vintage American collector-gun premium skewing that a bit, but it suggests the perceived quality (at least from a collector perspective) between the best Parkers and a Brit "best" is not hugely different.

Such a long, long time to be gone, and a short time to be there.