It's the Hokey-Cokey!

In?  Out?  Shaking it all about?  No 10 shows a bit of leg.  The Guardian reports:

Downing Street has ruled out involvement in a customs union with the European Union amid confusion over government policy as Theresa May prepares for a crucial week of talks.

After the exposure of divisions between ministers over the UK’s future relationship with the EU, an official source said: “It is not our policy to be in the customs union. It is not our policy to be in a customs union.” The statement went further than May who, on Friday, refused to rule out involvement in a customs union when questioned during her visit to China.

The development will anger remainers who have clung to hope that Britain will strike a deal with the EU that allows a close relationship with the EU after Brexit. But it will soothe the fears of Conservative Brexiters who have been threatening a leadership challenge if May moves towards an agreement with the EU that restricts the trade deals the UK can seek with third parties.

The clarification came on the eve of a visit to Downing Street by the EU’s chief negotiator, Michel Barnier, and as officials in Brussels prepared to begin talks on the transitional arrangements. Key cabinet colleagues will meet on Wednesday and Thursday to decide the details of the government’s policy regarding a customs union.

The Downing Street source claimed that a customs union was entirely different from a customs arrangement, which would allow the government to strike trade deals with countries outside the EU. The source also claimed that there had been no change in policy, saying the statement was a reiteration of policy outlined in a paper published in August.

There is an essential mismatch between the UK's desire to make third party trade deals and its putative customs "arrangement" with the EU.  The two options are incompatible.  You cannot allow goods to flow freely across borders with the EU and, at the same time, allow similar arrangements with third countries.