Meanwhile, in a briefing to the ambassadors of the 27 EU memberstates, Sabine Weyand -Michel Barnier's deputy- referred to the Custom Union as the basis of the future relationship and she's right of course: this CU is to be part of the Brexit treaty and forms the backstop, the absolute bare extremum beyond which there will never be divergence between the UK and the EU for the sake of Northern Ireland, meaning that anything that will replace it, must by definition be of a higher level of integration.
In short: forget all the fetish talk by Brexiteers about whether or not the UK can theoretically step out of the CU unilaterally, later!
The CU is not going to be a separate treaty signed after Brexit, its to be embedded in the exit treaty, so they are just wasting everybody's time with those discussions really, either because once again they demonstrate not to know how the EU treaties are constructed and relate to each other, or because they hope their voters don't and they can keep on fooling them to save their own career.
Note also how a full CU came into being: the UK itself (!) reluctantly asked the EU to extend the proposed customs regime for NI to the whole of the UK to avoid a custom border down the Irish Sea, yet the proposal also forsees NI remaining "dynamically aligned" with the EU's Single Market....
By the same British logic that opted not to go for an internal custom border, a intra-British regulatory border will not be acceptable as that is ever more complicated than setting up a few checkpoints in a handful of ports, which will mean this dynamic SM alignment is most certainly to be extended to the whole of the UK too then, later, during the negotiations which will start in April next year....
"Dynamic alignment" is a fanciful name to describe a legal mechanism in which the UK can not change its own rules to make them diverge from the current EU rules, yet will automatically follow any changes in EU rules..
In short: Britain is signing up to Norway plus (i.e. as good as full SM alignment and a full CU membership on top); it's just doing so in phases after realizing at each of the steps that none of the fanciful alternatives to each of the individual issues is workable: it could have made things easy for all and just asked to rescind its voting rights, because when all is said and done, nothing else will have changed materially, other than the colour of the UK passport, indicating second class European citizens by choice.
Another piece of info not mentioned in the British press today too obsessed with the CU end state: continued fishing rights for EU vessels will be included in the political declaration too... it's one of those small technical guarantees that lead to major policy shifts later on, just like the guarantee not to have a hard border in NI last year now does.