In this the age of "stuff," it seems almost inconcievible that the very concept of interchangable machine parts entered the world less than 175 years ago. Eli Whitney showed in the 1840's (as I recall) that guns could be built with interchangable parts. From that point on, Americans focused on factories that mass produced interchangable parts. The American paradygm was mass markets for utilitarian goods at low prices. The old European paradygm, on the other hand, was limited production of high quality goods at high prices. While the interchangible concept was not entirely wasted on Brit gunmakers, they had a had a tradition of small shops and believed their market was limited.

Seems that the Birmingham factories started to grow in the 1870's. It looks to me like that by around 1900 the issue of bench built mid-price and lower guns vs factory built was decided in favor of the factory. In this same time frame, the style issue of boxlocks and sidelcks got decided in favor of the boxlock as the high volume gun and the sidelock as the high quality gun. The higher the quality, the more bench time required. The more bench time, the more possible the small shop. Machine tools were expensive and hungry. Small shops couldn't afford a lot of tooling, but could afford to buy machined work from the trade. Same applies to specialized out-workers.

Always view the Brit gun trade with the understanding that all involved did whatever was necessary to turn a GBP.