If the supply is for a consideration not consisting or not wholly consisting of money, its value is such amount in money as, with the addition of the VAT chargeable, is equivalent to the consideration (VATA 1994, s. 19(3)).
Often, that is market value but not always and those words were removed from UK law in 1992 following criticism of them in a case in the ECJ.
See ¶10-105 for commentary on introducing customers.
Value of non-monetary consideration
If the consideration for a supply of goods consists partly or wholly of non-monetary consideration: