I think 2019 or maybe 2020 because as the polls get worse for the Conservatives, they will inevitably start suffering lots of by election losses, maybe even defections at this rate considering how divided they are to the point where they lose their majority even with the DUP and lose a vote of confidence.
Wouldn't it depend where those by-elections are. I think there is at least 150 if not 200 safe Conservative seats they will win no matter what but certainly with marginal you are right. Also with Sinn Fein's abstention policy it will be interesting if that impacts them since if those seats went Social Democratic & Labour instead the Tories would have a slimmer hold. Could there be pressure on Sinn Fein MPs to take their seats if the Tories lose 3 by-elections?