Again, let me say.
I don't know much about GB or the political landscape. I'm just asking.
Is it possible for Labour to campaign on a promise to hold another referendum?
I suspect it's highly unlikely. The EU will probably say, "Stop fucking us about. It's been five years - hard Brexit it is".
The EU has no incentive to accommodate the UK's stupidity and pretty much every reason to make this as difficult as possible.
Both major parties went to the last election with 'Leave' as part of their platform; Labour is highly unlikely to switch to a 'Rejoin' position unless that becomes massively and undeniably popular with the voters (at which point both major parties would likely adopt it as policy).
Brexit is a good example of the major problem of two-party politics, which depends for its utility on the parties taking opposing positions on major issues. The Liberal Democrats were the biggest 'Remain' party, but had little clout under the First Past the Post electoral system in the UK.
Regardless of what the parties and the voters might want, the fact remains that (despite not yet being fully implemented), Brexit is now irreversible by the unilateral action of any future UK government - to rejoin now would require agreement and unanimous approval of all 27 EU member states, any of whom could veto any application for membership from the UK.
And even if all 27 EU members were to agree to allow the UK to rejoin, the application would require compliance with the rules for new members, which are considerably more restrictive than the terms of the UK's membership before Brexit. The special status of the UK within the EU, which previously allowed them considerable freedom to opt out of EU laws and regulations, is gone forever, and can never be recovered. Rejoining would, for one very significant example, require the adoption of the euro and the end of the Pound (and of the fiscal control and economic independence of which the UK was previously very protective).
That's the real killer. The thing that makes Brexit such a hideous error. The very things that the 'Leave' campaign lied about - the idea that the UK needed to leave the EU in order to have full control of her destiny, and to avoid loss of sovereignty, are things that the UK
already had as an EU member, but almost certainly cannot have as a new applicant should she rejoin the EU at some future date.
These morons voted for something that damaged the country, in return for things they already had - and that they are now for the first time in history at genuine and real risk of having to give up at some future time. Brexit not only didn't gain any freedom or sovereignty for the UK; It actually puts their freedom and sovereignty (that they always had) under threat, should any attempt be made to reverse the massive economic damage done.
They've broken it; And no amount of apology or contrition will ever put it back as good as it was. Even in the unlikely event that the fuckwits who broke it could be persuaded to understand that they owe the rest of their country an apology, it can never be restored to what it was before.