"With Labour 10/11 to have the most seats at the next General Election, [...]"
There is a dilemma for many "remain" voters. They tactically lswitcheded their votes in the last election to Labour on the basis of "ABC" (Anyone But Conservative). Corbyn has since resolutely told his MPs to support - or not oppose - the government's Brexit agenda.
Those loaned votes are therefore not guaranteed to be his next time. Those voters can also be "idealists". Many will vote again for a small "no-hope party" of their preference - as being their only moral "choice" in an otherwise "no choice" situation.