Where is the evidence to say Nissan exiting the UK is happening, and would be utterly and solely to do with Brexit.
No. It would be due to economic conditions changing in general. EU tariffs might be countered by cheaper British labour caused by a recession or augmented by rises in the prices of imported raw materials caused by the collapse in the pound. It's a complex equation but Brexit is certainly not going to help and tariffs might be the tipping point.
Tarrifs on British made cars could be countered with tarrifs on cars made in France, Germany, Italy, Spain, Czechoslovakia
This isn't the school playground. Every tariff applied reduces World prosperity by inhibiting trade. Tariffs are bad. That's why people do trade deals. That's why the single market exists.
And if you think BMW are going to be hurt by us applying tariffs to their cars in the same way as Nissan workers in Washington when the factory moves to France, you are seriously deluded.