A good break down of the other polls done by Yougov on this subject by youtuber Phil Moorhouse.
My personal opinion is that Labour do not want to talk about Brexit until they can stop the Tory media telling lies on the subject. We will only get one shot at reversing this fiasco, so they want to get it right. With Labour I agree on this. Let it lie until we can have some actual facts on the matter. Any discussions with current media throwing in their bias is swayed by those who have pocketed billions from the Misery imposed over Brexit.
That one third estimate seems very consistent for a lot polls I am seeing this year. I put it down to the audience who listen and accept the lies of GBnews, TalkTV and the multitude to far right paper media. My opinion is that a third of the country are living in an echo chamber of bullshit. They are convincing the turkeys to vote for Christmas. We need a Leveson enquiry 2.0.