Unionisation didn't bring down British Leyland, the management did. There was no upward mobility from the unionised shop floor up to the management and design departments and it led to designs and management strategies that were completely divorced from both the realities of building cars and what consumers wanted to buy. The unions were a convenient scapegoat for Margaret Thatcher, but the reality is that Ford of Britain, Vauxhall, Nissan UK, Honda of the UK, and Toyota UK workers were all part of the same union but they didn't have relentless industrial disputes because they were managed effectively by people with actual experience in car production.
British Leyland were extremely reluctant to produce clean-sheet designs as well to save on R&D, which hampered their production capacity because production techniques were moving on from things that were complicated to assemble and maintain towards simpler assemblies which took fewer manual operations to put together. The Morris Ital and Austin Metro launched in 1980 fitted with an engine designed by Austin Motors in 1951 with the Ital inheriting a crudely widened (with deleterious effects on the handling) and very anachronistic front suspension assembly designed during World War 2 by Alec Issigonis debuting in 1948 on the Morris Minor while Vauxhall and Ford were selling cars with McPherson struts in the front and a twist beam in the back like you'd see in a modern car. The shop floor workers saw the problems, but they had no agency to correct them. The industrial disputes were a symptom of deeper rot.