In answer to a question on Facebook (quoted in the next line)

"The Remain-backing Mayor of London said Sir Keir Starmer should go into the next election with reversing the democratic decision to leave the bloc on the ballot paper "
If Labour went into a General Election with that as one of their pledges, (I believe that) they would automatically alienate a large percentage of their voters.

Maybe one day we will see an election paper with referendums (referenda?) included:
1, Which party should form a government?
2, Should there be a death penalty for certain offences?
3, Should Wales/Scotland continue to have a devolved government?
4, Should the aged have a 'right to die'?
5, Should the UK try to rejoin the EU?

But for now, the general idea is that in a General Election you vote for the person to represent you locally, not the party. Although many (possible the majority) are voting for the party.
And, if you do vote for the party, you are (assumed to be) voting for the ideas they put in their manifesto.

If referendums were included on the ballot sheet, maybe it would increase the number of people who thing that it is worth voting!