One of the many problems thrown up by both the 2016 referendum result and the 2017 general election result is that there is no clear majority for any form of Brexit deal: there is no parliamentary majority for a soft Brexit, no parliamentary majority for a hard Brexit, no majority to stop Brexit, and no majority for the considerable outlay in terms of public spending that planning for the chaos of a no deal Brexit would entail.
But there is still a fairly comfortable cross-party majority for a Brexit that takes the United Kingdom out of the political structures of the European Union but retains British membership of the customs union and single market. It’s just that because MPs of all parties are reluctant to vote against their respective party whips, that deal cannot pass through the House of Commons.
The trouble is that the Brexit problem is not on the Labour side. (Or, at least, not yet.) Although Jeremy Corbyn is a Eurosceptic, he is not an especially committed one and would – and indeed, has – changed his Brexit position in order to defeat the government. But there are too many Labour MPs who would vote against staying in the single market to make that gambit worth his time. The problem is that there aren’t enough Tory rebels to save the United Kingdom’s single market membership. That doesn’t really change if there is a Labour breakaway.
There is a “but” coming, though: if somehow Theresa May can cobble together a deal with the European Union, she will almost certainly need some votes from outside the Conservative Party to carry it. Labour will of course be desperate to facilitate a general election and the expectation in the party is they will vote against the deal. A breakaway party may be less keen to facilitate an early election, particularly if they have limited resources, or if they have flopped in the polls. And at that point, 15 or so breakaway MPs could be very important.