VAT is can only paid forward if the value of the goods are deemed to be under £135 in value - i.e. the seller charges you the VAT and has to pay that to the UK Tax Authority.
For goods over £135 in value, these are subject to VAT and Customs import duty, and have to be paid before the goods are released. This is normally done by the seller providing an invoice value to the carrier, who then calculates the uk vat and customs duty due, and you pay that to the carrier before the goods are delivered. i.e. If the shipper is say, FedEx, they will call you or message you saying you have £xx to pay before they will release for delivery (Royal Mail etc all operate a similar system)
tbh, it’s a bit of a minefield, the duty rates are variable, depending on what the goods are, and the carrier can add its own fee for handling the administration associated with all the extra paperwork. Bottom line, purchase price, plus VAT plus Duty, plus shippers admin fee can take you over the UK purchase price…..