The main difference is a factor of 10 in price. Traditionally, one would think the American guns are more machine made and the Brit guns are more hand made, but I doubt if there is much difference. For example, Purdey uses Hoenig machines for stock work. If someone insists on hand cut stocks, they sent the work to David Trevallion (ex-Purdey) in America. David charged about $6500 for the stock job. Butch made his own turning machine and finished and fits by hand, like most Purdey guns. Butch then glasses in for perfect fit and protection from oils and moisture intrusion. Purdey doesn't and this is a plus for the Searcy gun. Butch uses a stainless action (built from scratch, not based on a 20 ga shotgun like the Merkel or Rigby) which is also a plus. Butch makes both boxlocks and sidelocks.

The bottom line is that strength wise, I cannot see the Brit guns being as strong as Searcy, but H&H and Purdey are the classic names and superb guns. To each his own and viva la difference.