Alasdair Taylor's answer to Can VAT be charged after a contract has been made for spectacles? (115101972)

Certainly in the case of a B2C sale of a product, if an English law contract is made for an agreed price, that price should usually be assumed to include VAT.

This is the case even where the sale is outside the scope of the Price Marking Order 2004 - for example, because the products which are supplied in the course of the provision of a service.

One issue here may be evidential. If you have any documentation specifying the price, that will certainly help.