As I have commented before, it is worth avoiding the assumption that Brexit alone is the cause of lower growth forecasts.  The OBR has set out their assumptions of how growth will be reduced by leaving the EU.  They have also set out wider global trading conditions.  It is worth reading what they have to say on this:

"The global economy remains subdued, posing continued challenges for the UK economy. Global growth was 3.2% in 2015, the slowest pace since the financial crisis, and the International Monetary Fund (IMF) forecasts that global growth will remain modest, at 3.1% in 2016 and 3.4% in 2017. Advanced economies grew 2.1% in 2015, while emerging economies grew 4.0%, the fifth consecutive year of slowing emerging economy growth. China’s growth has slowed, as policymakers seek to move to a more sustainable growth path, but, like in India, growth remains well above the global average. Growth in global trade has also slowed in recent years to a rate well below its pre-crisis trend. World trade has grown at an average annual rate of 3.2% over the last four years, compared with an average annual rate of 7.1% in the two decades before the financial crisis. "

Their forecasts still shows a bigger impact of Brexit.  But the point to watch in the future is the combination of these factors - together they create layers of uncertainty and a bigger cumulative effect.