VAT was introduced in 1973 at 10% (reduced later to 8%) as a requirement of joining what was then called "The Common Market". In theory, post-Brexit, we could do away with it, or change its rates, but I expect the Treasury will want to keep that enormous income stream, and I would not be surprised if May's "Deal" includes some undertaking by the UK to retain it. I don't hold out many hopes of us being like other non-EU countries (e.g. USA) buying from the EU, where VAT isn't charged at all.