"Britsh Shotgun" I/II are definitely an excellent choice. I recently sold a pair on the BBS 4sale list as part of my library thinning.
Be aware that the volume2 has become especially rare and costly (aprox. $300 IF you can find one), while vol 1 is less so.
Other possiblilities could be "heyday of Shotgun" by DJ Baker. Concentrates mostly on 1800's > Ca 1900+ british guns.
Crawford & Whatley produced a nice little book "the History of W&C Scott Gunmakers".
"Birmingham Gunmakers" by Tate is a good overview of that particular historical end of the english trade.
The Gough Thomas "gun books" I/II, various Boothroyd's books (Sidelocks & Boxlocks, The Shotgun, Shotguns & Gunsmiths, etc) or even Mcintosh "Shotguns & shooting", "More Shotguns & shooting", "Best Guns" (Not exclusively English and not quite as technical or in-depth) are possibilities.
"shotgun technicana" by Mcintosh & Trevallion is an excellent techical reference. That is also getting to be difficult to find with correspondingly escalating prices.
Of course, a collection of the Double Gun Journal is always a great reference and has some of the best "Gun Porn" in print