For comparison, Remain advocates predicted a fall in GDP of 3.6-6.0%:
https://www.gov.uk/government/speeches/brexit-would-put-our-...
https://www.gov.uk/government/speeches/brexit-would-put-our-...
https://news.ycombinator.com/item?id=48473246
The actual data doesn't support any of these estimates.
Note the response of the guy I was arguing with. Faced with the reality that the sources he/she trusted are lying, they rejected the honest source they asked for because of "bias". This kind of motivated reasoning is everywhere in this debate. Quite annoying.