Many seem to feel that Brexit is somehow irreversible because “the people have spoken.”
While the UK, of course, has no written constitution, making this all much more complicated, there actually is a clear constitutional and legitimate path to reverse Brexit.
If a party were to campaign on a promise to hold a second referendum on Brexit, and that party were to win the next election, and conduct a referendum, then there would be no doubt about the legitimacy of the process and result.
I would even go further and say that were a party to campaign on a promise to only enter a coalition if its partners promised to conduct such a referendum, and that party’s votes were needed to form a coalition, then the result would be nearly as legitimate.
The problem is that neither of the remaining “national” parties is sufficiently united to run such a campaign. The Liberal Democrats could, but they have such little credibility after the coalition debacle that their path would be uphill. The Tories are here out of the question, which leaves Labor. Only two paths here, either the willingness to take the risk of losing a major slice of their supporters in return for the hope that they would pick up the liberals who defected to the conservatives last election, or Labor splits, and one wing links with the Liberals to form a clearly EU party.
I suspect that the last option is the most practical.
There is a point about timing. Really that is in the control of the Government. If whoever the new PM starts post-Art. 50 negotiations soon, then things might be well advanced by the time of the election, and a firm stance from the EU would raise to costs of secession for England and the UK. If the election were to happen before the negotiations, a lot of uncertainty might be avoided.
In any event, time is needed for the pro-Europe group to solidify for this campaign.
P.S. One plank in the platform would be allowing folks like me who have been away from the UK for a long time to vote, because we might lose our right to live and work in the EU. The failure to do so is one the factors tending to de-legitimate the prior result.