How would Britain fare outside the customs union?

The Irish border aside, it seems that Britain is not geared up to a life outside the EU. The following report from the current Private Eye is worrying:

A complaint issued by the European Commission shows how woefully unprepared Britain’s tax authorities are for a post-customs union world.

Brussels reckons that the UK owes it around €2.7bn in compensation for failing to spot imports of Chinese textiles at fraudulently low prices, according to a report from MPs on the Commons European scrutiny committee. Examples given by the Commission included women’s trousers declared at around 3% of their true value to dodge duties that, under the existing customs union, are shared around the EU.

The EC described the UK as “the most significant hub for this fraudulent traffic” run by “organised crime groups”. Worst of all, “in contrast to the actions taken by several other member states to fight against these fraudsters, the fraud hub in the UK continued to grow”.

[…]

Losses to the UK itself will become even more serious once the country leaves the union because, in the MPs’ words, “undervaluation of imports would present a direct loss to the exchequer as customs duties collected by HMRC would be retained entirely by the government”.