Maybe my history is a bit muddled, but as I understand it, the rapid collapse of the British motorcycle industry caused the UK government to offer bailout money to Norton-Villiers and the BSA Group if they merged and rationalized.
In this rationalization, it was decided to end production of the BSA brand, Triumph production would be concentrated at Small Heath, and Norton production would be based in Wolverhampton. With news of the impending closing of Meriden, the workers struck, and since Meriden supplied parts to the other factories, Wolverhampton was hamstrung and Small Heath essentially shut down, except for a trickle of Tridents. A new government came into power in the UK, the Meriden Worker's Cooperative was recognized and given money. NVT, or Norton-Triumph by that time, subsequently had a loan recalled and their export credits non-renewed. Norton-Triumph went into bankruptcy and was finally liquidated in 1978, with Meriden given the rights to sell under the Triumph brand. Meriden finally went into receivership in 1983, with assets being sold off in 1984. John Bloor bought the company, licensed Les Harris to continue building Bonnevilles for 5 years while he built a new factory in Hinckley, then started selling modern Triumphs from 1990 through the present day.
As I understand it, that's the collapse of the industry in a nutshell.
It's amazing to me that an industry would go from 30+ brands to 3 in 20 years (don't forget AJS and their Stormers)