Amid all the talk by the Chancellor about the Brexit dividends, one could forget that the OBR has updated its assessment of the economic impacts of Brexit:

1) Import and export intensity reduced by Brexit, consistent with the OBR's 2016 assumption of a 15% reduction in each

2) Trade volumes down, estimated 15% lower than had we stayed in the EU

(The OBR cites the analysis that @JohnSpringford has done isolating the effects of Brexit)

3) The OBR notes (rightly) that, because the UK-EU TCA is yet to be fully implemented, trade barriers will "rise further" and "probably take several years to come through"

4) Most importantly, the OBR claims that Brexit will lead to a 4% reduction in long-run potential productivity.

In other words, Brexit will have made the UK poorer by 4% in the long run.