The story is this:
British bike makers thought: "Coo! We'd better do something to counter these Japanese motorbikes!"
They thought about it and decided that the best thing they could do would be to abandon the lower capacity (cc) market to the Japanese, whilst they would build massive 800/100cc bikes.
The flaw in their plan was that in so doing they at a stroke wiped out any chance of British youngsters starting out on British bikes! They forced them to buy lower capacity Japanese bikes.
Now, if you buy a Japanese 125, learn to ride and want something a bit bigger, you would buy a Japanese 250. And of course, once you'd passed your test you'd buy a lovely British 800/1000cc bike, wouldn't you? No. Probably you'd stick with a Japanese bike. Go up to a 500cc and maybe go up to 800/1000cc a bit later.
The bottom did not fall out of the UK bike market, the management wilfully kicked it out.
The British bike makers struggled on so you got vile monsters created like the Tribsa (Triumph/BSA) but it was all down hill on a greased course of their own making.
A cousin was the president of a chapter of Hell's Angels. He told me that the rule was only British bikes and no Japanese bikes at all.
Eventually as British bikes became so scarce they had to abolish that rule and allow people to join who owned Japanese bikes. He wasn't happy with that, but he realised it couldn't be helped.