The Treasury's numbers are based on a scenario of Britain coming to a limited Canada-style free trade agreement with the EU, which would indeed be a poor outcome. The UK is far too deeply economically integrated with the EU for such a limited trade arrangement to work. But this is very unlikely to be what does happen. It is only likely if we think the government and civil service would seek a post-Brexit deal that they themselves believe to be against Britain's interests.
The best and most likely arrangement for the UK after Brexit would be to remain inside the Single Market – the so-called "EEA Option" – while disengaging politically from the EU. By maintaining economic union, at least for the foreseeable future, the UK can take the economic risk out of Brexit and voters can focus on the real question: whether political union with the rest of the EU is right for Britain and that the House of Commons would never vote through.
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