GOV.UK
When a Stamp May Be Applied: Accompanied personal acquisitions of goods from other EU Member States which are duty paid in the other Member State
Provided these goods are for personal consumption, no UK duty is payable on them and the goods must not bear a duty stamp. The offences of possessing, transporting, or displaying unstamped goods do not apply in these circumstances. However, if such goods were to besold in the UK they would be liable to seizure as they would not bear a duty stamp.