Hi David,
I think Blackmore, Bailey, Neal, and Back have written extensively about some of the pieces of the puzzle you described. There probably are a number of folks on this board who are better prepared to discuss this with you than I am but I'll give it a go. I think 3 factors are very important, the Tower armoury, the London Gunmakers guild, and the fact that most successful gunmakers made their fortunes filling military contracts. I think very few gunmakers only catered to local civilian customers. Most either built military guns or filled contracts for the East India Company, the Hudson's Bay Company, or export. The military gun trade was well organized by the Tower. It was also organized and aided by the gunmakers guild. The gunmakers guild helped to organize the piece workers and gunmakers to fill contracts, the Tower bought the completed guns or parts that were assembled in the Tower armouries. There were a lot of gunmakers in England, which also helped supply. There were inevitable tensions among all players about prices, supply, and demand, but I think overall it was a pretty efficient system. Also keep in mind that technology didn't change very much during the 18th century so the Tower would assemble guns from old parts that they saved from previous orders. I am sure that helped production considerably. I guess you could think of it as a decentralized factory system organized by a lot of tightly controlled middlemen. As far as iron and steel go, the British government probably purchased a lot from Sweden and Germany. Remember, the king had very strong ties with parts of Germany. They also were gathering resources from throughout the world as their empire grew. The use of patterns, pattern pieces (examples of firearms made to specifications and supplied to contractors) and standardized jigs helped to standardize components and firearms making production easier (but not capable of truly interchangeable parts). Anyway, that is my take on your question, a very good question indeed.
dave