They could get another 14 million if everyone who voted remain signs their petition. The point is, they lost. If Britain allows a do-over because the losers sign a petition, their freedom is gone.
There is no validation for the petition, so non-citizens could be signing as well. Mere numbers, while impressive as a shock value of such wimpy distress, mean nothing as the voting is DONE!
Still, looking at it, if the REMAIN had won, I do not think you would have had the BREXIT go off the rails like this as there is a definite difference in mentality at work here. Like our precious academic snowflakes, there seems to be a failure to accept reality here.