Springford compiled the hypothetical data by looking at countries which had a similar economy to the UK prior to Brexit, such as the US, Germany, New Zealand, Norway and Australia, and creating a “doppleganger” projection of where the UK could be right now.
Article source: https://www.huffingtonpost.co.uk/entry/brexit-cost-uk-government-revealed_uk_63a2d473e4b0a13a950ba3b5