While VAT is sometimes depicted as if it is imposed by the EU upon the UK, in fact the UK has consented to all VAT laws, since law-making in this area is subject to unanimity. The rule currently appears in Article 113 of the Treaty on the Functioning of the European Union:

The Council shall, acting unanimously in accordance with a special legislative procedure and after consulting the European Parliament and the Economic and Social Committee, adopt provisions for the harmonisation of legislation concerning turnover taxes, excise duties and other forms of indirect taxation.

Source: EU Law Analysis: EU Referendum Briefing 1: Can the UK control the EU’s future if it stays a member?