We will have to watch and see, the UK government gave Nissan and Honda insensitives to open there plants here so i dont think any government in Europe are alone subsidising car plants or any other manufacture for that matter, it makes sense to do so because of the jobs etc it brings.
As far as tariffs hurting the EU more than the UK i would agree with you 100%, but as the main bank roller to the EU is germany and they are the keenest to see the EU model be successful i think they will take the hit on the nose that tariffs will bring, after all they absorbed the cost that taking the east into there fold and came through it.
Of course it must not also be asumed that because your UK BMW may cost 10% more thay will stop selling in the UK, it just means they will sell fewer so the economic hit will not be as great as a complete cut off of trade, of course if we lose all the volume car manufacturers in the UK due to relocation then it will just mean we pay more for any car, also a fair proportion of the UK v EU trade gap is car production, so if the big boys were to relocate a fair amount of that gap would disappear