What happens post Brexit? The borders come up. Even if they are simplified, the UK will either be part of the customs union or not
If not, then customs is needed for every single thing.
If it IS part of the customs union then it invalidates what the UK wanted - it wanted to reject the 4 freedoms (of goods, services, people ad capital)
Is this understandable to you?
Think if Virginia leaves in the same manner as no deal Brexit. Then it needs to be part of the US customs union or not. Brexit wants out.
Will there be new a different import duties between the UK and the EU post Brexit? Probably yes. Is it the end of the world hell no. A blip.
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