Because British nationalised industries have such a stellar success rate. You need only look at British Leyland. Or British Steel. Or British Coal. Or any of the other countless industries that no longer exist in Britain after they were nationalised, merged into one large company and then spectacularly driven into the ground by stunning incompetence from everybody involved; governments of both sides, management and unions.
For that idea to work the first POD would have to be 'Post-war British governments are capable of competently running a nationalised industry'. Given most of them weren't capable of organising a booze up in a brewery while holding a copy of 'How to organise a booze up in a brewery for dummies' that's quite an ask.
Not to say it's not possible to have effective nationalised industries, France and many other countries have and do. Just that successive British government have lacked the skills/willpower/whatever to consistently do so and I have a feeling that a POD early enough and large enough to effect such changes would butterfly away any recognisable post-war situation.
The problems in the British motor industry were mainly in the volume car end, Britain had many successful makers of upmarket and sports cars and commercials. When BL was founded in 1968 some of its constituent parts such as Jaguar, MG, Leyland Truck and Bus, Rover and Triumph were leading brands in their sectors, the theory behind national "champion companies" such as BL was that the good companies would strengthen the weak because of their better management. Instead the opposite happened, the successful brands were starved of investment because of a political imperative to keep the Austin-Morris volume car business churning out abominations like the Allegro, Maxi and Marina. The former British Motor Corporation was an absolute quagmire that I doubt Henry Ford could have made a success of, BL's senior management such as Donald Stokes and Harry Webster were from a heavy trucks and sports car background and didn't seem to understand the volume market.
What the Government should have done was what happened with GM and Chrysler, split off the good from the bad and ensure that what's viable isn't dragged down, however the mass redundancies this would have entailed was anathema in a Britain that was committed politically to full employment at all costs.