Labour wouldn't be impartial in the subsequent referendum. Part of the problem is, as Gemmill said, the Tories and Lib Dems have very simple clear positions. Labour's position has not been in any way clear ever since the referendum and is continuing to be unclear into this election. There was program on BBC last week with some hairdresser from Wales moaning that she's pro-Brexit and would never vote Labour again. McDonnell was then asked about it and he had to toe the party line and waffle, offering absolutely nothing. If he was allowed to come out and say, 'I'm sorry she won't vote Labour but Remain is the best thing for our country' then they'd win votes that way. As it is they're trying to be all things to all men but coming across as nothing to nobody.
Revoke is a nonsense, the Lib Dems can claim that position because they're essentially a single issue party with no chance of a majority but democratically it would be the wrong promise for Labour to make.