The EU Customs Union is a relic from the 1950s. It is redundant.
The world beyond the EU, where there are no significant customs unions, has moved on.
As part of the EU Customs Union (which also includes a non-EU member, Turkey), the UK does not “own” the customs duties (also known as “tariffs”) it charges on imports of goods into the UK originating outside the Customs Union. The UK merely collects the customs duties on behalf of the EU and hands them over to “Brussels”, less a “handling fee” of 25% of the amounts collected. EU customs duties are a tax on British manufacturers and consumers.
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Download Briefing Note: BN#70 Customs Duties