The UK language is consistent with almost every nation's duty structure. Items which you intend to permanently import are subject to an allowance and a duty. Items which you intend to export when you leave are not.

While, of the many things which can go wrong on entering the UK this is not one of them, if it worries OP, he should travel with the receipts for the items and be prepared to show those along with the items on arrival and departure. But, that is only because the receipts don't weigh much or take up much space.