From the beginning the sequencing adopted was designed to stop HMG from using divorce issues like the border as negotiating leverage when it came to the future relationship. I don't see the EU allowing it now.
If HMG got a legally binding agreement to a CU without an equally legally binding agreement regarding level playing fields that would be disastrous for the EU, so how could it possibly happen?
Yesterday's story in the Guardian about fisheries was a perfect example of how the EU would risk throwing away important leverage if it agreed to a CU now without ensuring that this would have to be negotiated to the satisfaction of both parties.
And all of this is ignoring the more fundamental legal point that the CU can only be negotiated after Brexit. And in a negotiation, both sides have to agree. So how can guarantees be made at this stage that the negotiation will be successful?