But a hit to german trading to the UK is a small price to pay instead of the hit it would take by setting precedent that you can just willy-nilly leave with all the perks. And I can tell you, the german-language media is already full of "priority number one if to stop a cascading copycat effect".
As a snarky side remark, what will the UK do in the future instead of continuing to import cars, regardless of terms? Rely on its domestic automotive industry?