It's a bit of a grey area. If you move here permanently I think you're meant to declare the values of everything which would attract import tax/duty. If you were ever to leave again you could get it back. Similarly, any goods bought here which are taxable can have the VAT refunded if you leave with them permanently (Heathrow airport has a desk for this, as do some shopping centres e.g. Bicester Village. It's very popular with Chinese and Middle Eastern tourists buying luxury goods.).
For a holiday there's no assumption of staying here permanently, so no tax. If you brought something in and sold it, and the amounts were above the threshold, then yes I suppose technically you should have paid import tax/duty on it in the first place.