From the study:
"As with all modelling approaches, E3ME is a simplification of reality and is based on a series of assumptions. Compared to other macroeconomic modelling approaches, the assumptions are relatively non-restrictive as most relationships are determined by the historical data in the model database. This does, however, present its own limitations, for which the model user must be aware:
• The quality of the data used in the modelling is very important. Substantial resources are put into maintaining the E3ME database and filling out gaps in the data. However, particularly in developing countries, there is some uncertainty in results due to the data used.
• Econometric approaches are also sometimes criticised for using the past to explain future trends. "
So this model is based on macroeconomics and past history. The whole point of Brexit is to deregulate the British economy by decoupling it from the nonsensical, Euro-centric and near-socialist view of regulation on the Continent. As has happened in the USA, this deregulation will lead to higher capital investment and consumer spending, and hence economic growth. This study is about as worst case as it can get, which is not surprising given it was sponsored by the Brexit haters in the London city government.