I don't know how they came up with their numbers but media outlets seem to be confident quoting their figures. Also I found this written by a Tory who was an MP and is now in the House of Lords:
The latest estimates are that Brexit reduced GDP by 6 to 8 per cent by 2025, with business investment down 12 to 18 per cent.
If I were to see news articles saying that Brexit is having a fantastically positive effect on the British economy then maybe I'll look at Brexit as being somewhat beneficial. But I haven't seen that. Also like I said, I think having close ties with Europe can have some benefits beyond just easing international trade. It means Europe together can stand up against big countries who dislike Europe and want to harm Europe.
Maybe we won't agree though. I guess you think Brexit is going to be a good thing for the UK in the long-term? I guess I'm not so convinced about that.