Why not join a customs union?

As Sam Lowe, a trade expert at the Centre for European Reform, has written: "The UK is an intermediary manufacturer - one that imports to export - and the EU is, and will remain, the UK's most important trading partner". Reducing border friction is a reasonable ambition.

But one of Labour's six tests is that Labour says it must deliver the "exact same benefits" as we currently have as members of the single market and customs union. This proposal would not do that.