I feel the EU want to punish us for daring to break up their club.
The EU have got to punish us. They can't be seen to be giving us a favourable deal which is free of the obligations of being a EU member. With regards to the free movement of goods and services, the outcome
has to be inferior. Even those non-EU countries within the EFTA have inferior deals which cover subsets of their economy, and pay a great deal for the privilege.
As for the Mini assembly, as far as I know, they operate a conventional system where components from both the UK and other EU sources are assembled to completion in Cowley. Certain specialised components may cross currently frictionless borders several times, but most components come from a single contracted manufacturer - this is how modern car manufacture takes place. Don't forget that we have some major component manufacturers in this country who export to other EU countries.
BMW also have a another Mini assembly plant in the Netherlands, VDL Nedcar in Born - the old DAF / Volvo factory. In fact
all Mini convertibles and Countryman models are assembled at Nedcar, as well as the BMW X1. The Nedcar plant currently has significant spare capacity.
The obvious answer for BMW is to continue final assembly of imported parts for the Mini hatchback at Cowley for the UK markets, thus minimising import duties to parts only, and expand production within EU borders for EU markets. I'm afraid that long term, if we find ourselves outside of the EU single market, this will become the blueprint for most car manufacturers which have facilities in both the UK and the EU, and longer term, for many other exporting manufacturers.
We're heading for a hard Brexit, so I guess we will get to find out how it all turns out in 20-30 years. One final thought - how many people think that plants like Burnaston, Sunderland or Swindon would even exist if the UK had been outside of the EU?