Agreed. Is this figure supposed to be high? Seems like there's a good chance that 1 in 5 Biden voters now think it was the wrong decision and 1 in 5 Trump thought similarly after his years in office.
People may regret their choices in elections, but they can revisit them every few years. Brexit is, if not forever, at least for a very long time.
Two mutually contradictory caveats:
1. The referendum itself was never intended to be serious. It was held because David Cameron thought it would fail, in order to shut up the anti-Europe wing of his party. He resigned over it.
2. Although the referendum was close, it was effectively affirmed by the elections in the wake of Cameron's resignation. It was still close, but they ended up with a majority for the party campaigning explicitly on a pro-Brexit platform.
This adds up to a cautionary tale on referendums. A big question was put to the people -- even bigger than an election -- and a largish majority (not a bare majority) are unhappy about the result.
In that sense, yes, 20% is a pretty big number. Not that it changes anything. It's much too late for that, and it doesn't tell us anything about the counterfactual where the referendum lost. But it's worth remembering going forward, both with respect to the EU and for national decision-making.