Is Brexit a Big Part of the UK Economy?

The IMF forecasts aren't necessarily the best in the business, but clearly the UK economy has its own particular problem. The Office of Budget Responsibility says that Brexit will have a long term effect of cutting the UK's GDP by 4%. I recently read about £100 billion in lost output and an annual 40 billion loss of tax revenue to the Treasury every year. So yes, obviously it's not the only reason, you know, business investment has been flat since June 2016 what happened in June 2016 that would make businesses wary of the UK.