It's quite probable that the next general election will be after the UK leaves the EU. The next prime minister will probably be whoever wins the Tory leadership election this summer, since the Tories have an outright majority.

That's an option. I'm trying to check voter data against constituency data to see if it would be possible for remain voters to vote for a party with a remain stance and win a majority. Cameron was elected with 36?% of the vote so having 48% should be doable. You would only need to contest England and Wales as the SNP and Irish MPs would support the motion. It also keeps it all nicely democratic rather than overturning a majority vote we use the existing political system to have a mandate from the people to ignore the result.

> What if the next elected PM is elected on the promise of staying in the EU?

They won't win the election on that promise alone, and having a full-fledged campaign alongside this kind of promise is a HUGE gambit. Many people would be unwilling to risk a good chance at winning the election over one campaign promise.