Because we don't have the raw materials here. If you shut down the ports and airports, you could not construct a modern car in Britain even if you wanted to---many of the raw materials that are necessary are not found on the island.
Whether it is a very old pre electronics era car, or a modern one with fancy gadgetry, the main material used in a car is steel. Steel is mainly iron ore, one of the commonest ores on the planet, and I suspect it is still largely plentiful in Britain. Plus, with a more stringent, energy prudent, recycling regime - including recycling steel here instead of shipping it abroad - we can be self-sufficient when it comes to steel.
As for smaller parts and materials that make up a car, I realise that modern vehicles have become very complex - choc-a-bloc full of hi tech gadgets, rare earth metals, largely to be found in China etc - but we need to start to engineer simplicity in cars and a lot of other goods. Things that become too complex invariably become technically impregnable to localised, low tech, DIY repair, which is an essential to building in resilience in a post peak oil world. Those who profit from the throwaway society, things being ineconomic to repair, and making people reliant on expensive experts and imports to repair things don't like it, but it is a win-win situation for people, consumers and the workers.
Additionally I am reminded of something you said earlier about the impact of a coronal mass ejection. It - on a very large scale - is of course a very low risk, high consequence event, but were the earth to be hit by one currently, could not such a scenario lead to the frazzling of electronics in cars? And of course, out with the occasionally temperamental peccadilloes of the sun, there are other circumstances in which this can happen on a wide scale. In such a scenario, the only thing left capable of driving would be vintage cars like VW Beetles, Ford Mustangs etc. And of course modern vehicles that have been stored in Faraday cage lined buildings. But outside certain quarters and corridors, contingency planning like that is probably few and far between, and wouldn't be much use to ordinary people and businesses trying to get back to normal and navigate their way through some semblance of civilised life in such a scenario.
The analogy is clear. Supply disruptions brought about by war, man-made or natural disasters, the information revolution and the proliferation of 'surround sound' communication creating a 'megaphone' effect leading governments into knee jerk reactions, the end of cheap energy or a combination of all. Ultimately, on a slow burn, step by step, creeping change basis, the end result of such events, and flash sudden disasters mentioned in the paragraph above, could be much the same.
