Voters should have a say under proportional representation. Holding a general election under First Past the Post would likely hand the government to a faction with minority support. I don't mean this to be partisan or pro-/anti-Brexit, but FPTP typically doesn't settle issues and increasingly doesn't provide good government in the UK.

FPTP typically overrepresents some parties at the cost of others (and at the cost of voters). Politicians might want an election when they believe they will be more overrepresented afterwards than before. That's why it's no surprise for the opposition to suddenly stop wanting an election when the government begins to want one.