Non-monetary consideration: the law
UK law unlike Article 73 of the EC Directive 2006/112 contains separate valuation provisions to be applied to monetary and non-monetary considerations. The relevant UK provision is set out in section 19(3) of the VATA 1994:
If the supply is for a consideration not consisting or not wholly consisting of money, its value shall be taken to be such amount in money as, with the addition of the VAT chargeable, is equivalent to the consideration.
In other words, you have to determine the amount that would have been given in money for the supply if goods or services had not been used for payment instead. Once you have identified that amount, you should use the VAT fraction to establish how much VAT is due because it is calculated on a VAT-inclusive basis in the same way as if the payment had been made in money.