They are almost certainly assembled here: finished cars make a poor export item (because you are shipping a great deal of fresh air). And also there’s the whole right-hand steering wheel issue.
I don’t believe we have tariffs to protect one set of shareholders over another (e.g. Jaguar over Ford) - though of course I may have missed some recent dumbass directive from Brussels. And of course we have our share of Global Warming R-Tards who want us to drive electric shoeboxes.
But in general car manufacturer capital seems to be treated well in the UK. For instance I believe Tata are investing bigtime.
Actual car-drivers are treated like garbage (our fuel tax is, what, three times yours?) but car plants seem to be doing well here.
As a business model, Ford found it more profitable to build in the UK, which is one of the thrusts of my line of thinking.
A tariff would merely make it more profitable to assemble cars in the UK rather then have them imported.