I think it's an issue of logic.
The Brexit Party isn't going to win a majority in parliament. If you vote BXP in a marginal constituency and thereby enable Labour or the LibDems to win the seat, you're decreasing the likelihood of Brexit taking place.
So you've just stiffed yourself
Northern Monkey ... who can't upload a bleeding Avatar
No, because this is a GE not a ref', so why vote for the tory party if you believe that the other policy's they have go against everything you believe in. And unless I have missed something, the LIB DEMS have said they will cancel it, the Labour party have said a second people vote is they way to go.
Northern Monkey ... who can't upload a bleeding Avatar
Except of course that the "other policies" that the Tories are campaigning on include massive increases in public spending (popular with traditional Labour voters) while Labour are pledging to not only reverse Brexit but to keep freedom of movement (massively unpopular with traditional Labour voters).
So your assumption that former Labour and now Brexit Party voters would sooner return to Labour than vote Tory is not necessarily correct. For many of these people, the Tories now better represent them than the metropolitan liberal elite-dominated Labour.
Depends what 'getting Brexit done' means, and what a voter might want from it. If you're a fisherman who thinks the MayBoris deal is no good for them, or someone who believes that we should invest nationally in high-tech manufacturing, then the Tory deal is not as attractive as a Brexit Party WTO deal where we are really free to choose our direction.