Tony Blair has issued a rallying call for all of the Remainers. He has asked all of the remainers to block, which would be, a damaging Brexit. He is pushing for Britain to remain in the EU - despite the Brexit vote on the 23rd June.
I personally have met a lot of Regretters, that is to say remainers who regret voting for remain. I haven't met a single person who regrets Brexit, that is my own anecdotal evidence though of course.
He would claim that the people didn't know what they were voting for, or the consequences of such a vote.