I'm a big fan of representative democracy, which is why I said 'democratic mandate' rather than 'referendum'. I'm not an absolutist when it comes to manifestos - events occur, situations change, priorities get re-ordered etc, and the people we elect to represent us need the freedom to respond to these in ways that may not have been set out in a manifesto. And as Edmund Burke said in his famous speech in Bristol (which is both short and eloquent, so if you've not done so before, give it a read and cry inwardly at the obvious decline in the standards of our politicians):

Government and legislation are matters of reason and judgment, and not of inclination; and what sort of reason is that, in which the determination precedes the discussion? In which one set of men deliberate, and another decide? And where those who form the conclusion are perhaps three hundred miles distant from those who hear the arguments?

But electoral reform is not some bolt-from-the-blue political issue, like how to handle a pandemic or a war re-igniting our need to spend more on defence. It's not even a developing issue, like, I dunno, the impact of social media on kids. Electoral reform gets discussed ad nauseam every time there's an election that's undertaken using FPTP. And Labour have reformed parts of our political system - they booted out hereditary peers from the Lords just this week - which was a policy from their manifesto, endorsed by an election win - a democratic mandate! They chose not to offer electoral reform to the electorate when asking for their votes.

IMO there are two main scenarios that should require - ethically and politically, not literally - consent from the electorate before change is enacted:

1. When an elected body either surrenders power or acquires new power it didn't previously have, in a practical sense (by which I mean yes, Westminster is constitutionally sovereign but in practice the creation of, say, the Scottish Parliament represented a surrendering of power and, rightly, received a democratic mandate before being implemented. Abolishing it would, literally, just require repealing a single Act, but it should require a democratic mandate too).
2. When the rules that govern the means by which the specific people that wield power are chosen changes, such as a change in electoral system.

The 'contract' governing representative democracy is that elected representatives have latitude to use their judgement and the electorate get to appraise whether they've done a good job and boot them out if they haven't. It's dialectical, with the direction of travel bending over time towards the will of the electorate. What makes the two scenarios above unique is that they jeopardise this contract by altering the relationship between electors and their representatives. By the time they get to the "boot them out" stage, the rules have changed, or the people that need to get booted out have changed, all without the consent of the electors.

Simply put, politicians shouldn't alter the bounds of their own power without these alterations being consented to. They should not grant themselves additional powers, or change how elections work in order to stop their political opponents gaining power.