Who killed the British motorcycle industry?

No one, probably. Industries and societies are complex systems and undergo elaborate evolutions, and when something goes wrong it usually is not due to a simple reason. However, we can speculate that it parallels what happened with the British automotive industry. In 1950 52% of world exports of automotive vehicles came from the UK. Today whatever is left of the industry is foreign owned. This video has Jeremy Clarkson's take on the problem.