Why would you import cars form a factory in the EU when you can make them duty free in the UK? All the UK producers need do is lower their price a few points and gain a greater market share. That’s the whole point of an import tariff to make domestic suppliers more competitive against imports and to promote domestic industry AND IT WORKS!
Don't try to say "it would work for the US and it would hence work for the UK" -- the US can cut itself off from the world, while the UK, ever since the 1700s, could not