The UK wouldn’t be able to sign separate trade agreements on goods, but it would still be able to do deals in the service sector (there are no tariffs on services). The other thing worth remembering about a customs union proposal is that – on its own – it wouldn’t meet the test of avoiding a hard border in Ireland “under all circumstances”. It would help, but there are other checks that need to take place at borders that have nothing to do with customs. So a customs union alone also wouldn’t guarantee that trade between Dover and Calais, an economic lifeline for the UK, would be as open as it is now.