the Japanese companies are based in the UK for easy access to the European market, a hard Brexit would force them to move their factories to mainland Europe.
In the medium to long term, when major investment decisions are considered, I believe this is inevitable.
Realistically though, mechanisms will be found to allow companies to continue to operate in the short term, but it will be far from an ideal situation for the manufacturers, as Honda UK indicated. It may also cost the UK Government a great deal of money to facilitate this. I would pose the same question as I did earlier - Does anyone think the Burnaston, Sunderland or Swindon plants would have ever been built, had the UK never joined the EU/EEC?
We import almost 1 million cars per year from Germany alone, worth almost 30 Billion Euro (Germany imports about 4 Billion Euro of cars from UK) after Brexit our cars could become expensive in Germany, but German cars could also become very expensive in UK
...and we will continue to do so. Why should the EU care that we apply tariffs to their cars? If we did carry out the threat to walk away and assume WTO tariff rules, it will only hurt the British consumer. Prices of UK built cars will also be affected by tariffs on imported parts.
We could say stuff your cars and vans, but where else will we buy our BMWs, Mercs and Audis? Cars are not really a fungible good to many buyers, I doubt tariffs will significantly affect the habits of BMW buyers, only the prices they end up paying.
UK manufacturers have far more to lose from tariffs. Loss of sales of UK built cars will only increase sales of EU built cars, and incentivise manufacturers to make future investment decisions in favour of the trading areas where their biggest markets are. As I said earlier, companies like Peugeot already have spare EU capacity they could use for cars and vans they build here in the UK.