Also, perhaps it's my limited imagination but I'm struggling to imagine how you'd even craft a rejoin campaign this quickly where the central slogan isn't some variation of "we done *** up". Now, granted, a similar issue could be said to roll around at every General Election where the Opposition asks the Electorate to, in effect, reverse their decision of the last Election, but the difference in that case is that it's understood that the decision of who governs is under periodic review. The Brexit Referendum was, at least supposedly, meant to be a one-and-done issue (for a good 20 years, at least); and it was arguably reaffirmed emphatically at the 2019 Election (and even the 2017 one, since both main parties campaigned on a platform of honouring the result).
But anyway. The point then is that a "we done *** up" campaign slogan just won't cut it, but what better is there? The entire premise of Rejoin is that Brexit was a mistake that needs to be reversed ASAP, but, if so, it was a "mistake" that the British people made (twice). You don't win campaigns by telling the electorate that barely six years ago they made a monumental error. You have to wait until they come themselves to that decision.
By contrast, then, writing a campaign to stay out is easy: "Make sure they never stop listening to you". I don't even believe in this and I'm pretty sure even *I* could write a winning campaign to stay out of the EU right now.