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Depends how it was 'shipped'. At a sea port if it arrives by container on a boat. At an air port if arriving by plane.
(pre-Brexit) At the EU border if arriving by land but I'm not sure how much arrives that way?
Are you suggesting things are transiting through the EU in a customs sealed way (TIR Convention?) who no longer check it because it is exiting the EU and then the UK are not checking it either because it has come from the EU?
Which isn't going to have much impact on things coming from China that should already be subject to those checks at some point in the journey.