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I see Corbyn is out in Brussels talking about his deal. This seems quixotic given everything the EU has said.
Whatever deal he wants the WA will have to be a part of it. And indeed there is nothing stopping him approving it now except that he will have no control or influence over any longer term negotiations falling out from the political declaration.
May has some simple things she could do (or could have done) to get her deal to pass and achieve brexit But she's not willing to dump Davis, Johnson, Mogg and the other bone heads and give Corbyn a chance to prove he can actually manage anything. Maybe he doesn't want that anyway, would rather settle for looking heroic fighting against "tory brexit" as the country goes down.
Also she has cabinet members who would prefer no-deal to a customs union.