I wish the OP well in finding the gun he wants, and there is good advice in this thread. Happy hunting days ahead!
I'm chiming in only because the words 'British singles' appeared, and you can't have enough of that kind of eye candy. Canvasback, let's see more of that Tolley. I would rather spend $$$ for a handful of bismuth shells and bring down a bird with a gun like that, for the sheer pleasure of the experience, than fill my limit with a factory widget.
My British single is a 16 by Thomas Seymour of London (1841-1844), converted from percussion, weighing just short of 5 pounds (yes, it kicks something fierce):
And just for fun, here's a single Boss & Co. of London, one of only three single pinfires the firm made:
In its day, it would have cost someone's annual salary.
And a 12-bore by the Edinburgh firm of John Dickson & Son, one of only ten single pinfires the firm made:
Finally, a single 16-bore conversion by Thomas George Sylven of London, made with a barrel from a percussion gun by the London gunmaker Richard Seffens (1820-1829). The client must have prized that barrel to have wanted a new breech-loading gun built around it, either for its shooting qualities or sentimental value.:
OK, I'll go back into my dark cave now, and leave you all to your practical recommerndations.