VAT is charged on monetary and non-monetary transactions. A good case to look at regarding Barter trade cases is the Naturally Yours Cosmetics case – just google it. Generally VAT is subjective on such matters but value is based on what seller would have paid if monetary. Even if no cash has been paid if there is consideration, VAT is charged and is often not possible to defer VAT if goods are supplied. The tax point is point of delivery or <14 days’ invoice.
There are however other non cash transactions which VAT is not applicable like gifts below a certain threshold – currently £50.