One history that I've got somewhere records over 100 separate Motorcycle 'factories' in the greater B'ham area. The Most well known are, of course, the BSA at Armoury Rd, Triumph at Meriden, and Norton at Bracebridge St. James & Excelsior were at Tyseley , (now part of the I-Mex site), Velocette were at Hall Green, Ariel at Selly Oak, York Rd, but there were many others.
Ok Supreme, New Imperial in the Jewellry Quarter side of Handsworth, and a host of smaller makes throughout the City & suburbs. My Great Grandfather made the 'Metro' in a small factory in Ladywood, just down from the present Police Stn. Broad St and its off roads had about a dozen makes.
Most of these used heavyweight bicycles with proprietry engines, like the Villiers (Wolverhampton) to power them. Many started in the 1880s and 90s but folded at WW I . Some were reborn in the 20s and went through the 1928 Crash to emerge as combines. Again, WW II caused problems, many firms were lost, or moved to other production types. Even the major ones such as Matchless & AJS had to Combine in AMC, Later, due to appaling corruption amongst the management, even BSA, Triumph, Norton & Villiers ended up as one 'lump'.
Velocettes main problem was, as ever, idiots at the top failing to listen to the buying public. Whilst everyone was shouting for the Sports single, the Goodmans were talked into producing the LE (Noddy Bike) and Viceroy. Just as Ariel's disasterous foray into little enclosed machines, (Leader & Arrow) caused their downfall, the Velo drove themselves into the ground. Even BSA were'n't immune from this stupidity ... remember the 'Ariel 3' ?
The Classic and Vintage 'bike world is a very healthy one, worth £ Billions world wide and bringing over £ 1.1/2 Billion into the UK in sales and parts remanufacture alone. Average price for a 500cc Velocette Venom is about £6,000, with Thruxtons worth up to 3 times as much. LE's ? Unless you really want one ......