So, if you're a Labour member, and you vote tactically, that's you out...
(Personally, I'd think that if people who actively paid to be a member of my party had voted for someone else, my priority would be to get them to change their vote, not their membership...)
From - and Guardian coverage here.
Edit: I know Lib-Dem members who voted Labour at the last election, to keep the SNP out, and ones who voted SNP to keep Labour out. I don't blame either group. Under FPTP I expect people to vote however makes sense in an individual seat at a given time. (Unless they're focussed on the long-term view, of course.)
Original post on Dreamwidth - there are comments there.