topsy if you want to argue about Brexit impacta on the car industry, you need to go and do some proper reading. This superficial stuff about German car manufacturers is just irritating.
The trend is that the developed world makes less and less cars over time
This is very misleading. What time frame are you referring to? Are you forgetting the small matter of the financial crisis and its impact on vehicle sales in major western markets? What do you mean by the "developed world" - are you counting US cars made over the border in Mexico, for example?
One of the big changes in the car industry in the last couple of decades is that production of mass market vehicles now mostly takes place within or close to the market in which the car is sold. A decade or two ago, (within my working career as an economist/ analyst covering this market among others) many of the Japanese vehicles sold in America and Europe were made in Japan. Note that "within or close to" includes vehicles made inside trading blocs - Fords assembled in Mexico for the US market, Toyotas made in the UK for export to the EU, etc.
Cars are mostly made close to where they are sold. It's no surprise that China is now the world's largest car manufacturer, because it's also the world's largest car market in terms of sales - and it makes little sense to ship mass market cars halfway around the world. You can be extremely confident that unless the UK somehow develops a world-beating luxury vehicle manufacturing industry that UK-built cars are never going to be sold in China in any numbers.
For the UK car industry to thrive (and it's not just the foreign-owned car companies that matter: there are lots of UK companies selling materials and components to the car industry), it needs access to the EU market.
This is worth reading, on the subject of NTBs and the gravity model, and is particularly relevant to this thread.
medium.com/@Nightingale_P/ice-cream-selling-in-egg-land-10c4419f89d8#.c67cbye5w