Northern Ireland Protocol
Nobody wants the Northern Ireland Protocol, and the Pro-Brexit Unionists are making the most noise about the treaty which imposes a border down the Irish Sea, which Johnson had previously said no Conservative Prime minister would ever do.
World Trade Organisation
The rules of the World Trade Organisation insist that there must be a border between Customs areas. And that all Third countries, which the UK now is, must be treated the same. So what alternatives are there?
Joining EFTA may be a possibility? But would EFTA members want the UK as a member? And it’s the Customs rules that are the outstanding problem, not the Single Market.