Tony Blair is making it his mission to change public opinion in favour of staying in the EU. While I have issues with Tony Blair and his New Labour experiment, I'm with him on this. After all, the top Brexit campaigners all have controversial back stories too. In this new age of populism, having 'baggage' from the past seems no hindrance to high office, including the White House.
If Blair can change public opinion and there's a general feeling in the country over the next couple of years that the people no longer support Brexit, then there would definitely be grounds for holding another referendum. What would be unforgivable would be for Britain to leave the EU against the will of the people at the time, purely on the basis of a small majority vote in an advisory referendum 3 years previously where the Brexiters' campaign was based on lies and ridicule of the remain side.
What is really needed is for Sir John Major to come forward and lend his support. Two former prime ministers from opposing sides speaking together in favour of staying in the EU would be a powerful force. Whether making that platform big enough for four former prime ministers, including Brown and Cameron, would be a good idea, I'm not so sure. Brown is associated with political panic and chaotic government and Cameron is associated with that huge political misjudgement that led to this Brexit mess in the first place - the referendum.