"The popular will" is the kind of terminology typically used by dictatorships. I don't trust the term.

Governments don't operate according to "the popular will" - they operate by what seems like a good idea at the time, taking into account what they think might help to shape things so that they get elected next time (and what might help their political careers along the way).

Insofar as "the popular will" enters into the picture what matters is the actual picture at the moment, not back in the last election.

The last election, and any promises that may have been made in the manifesto or during the election, are past history, at times useful to dredge up for rhetorical purposes, but at other times best forgotten and obscured.

What led the Tory bigwigs to ditch Thatcher was not whether there was or wasn't wasn't any "mandate" for the poll tax in the previous manifesto, it was the explosion of anger in the streets, expressed in plummeting Tory votes in by-elections etc.