Why would digital art be exempt? *exempt is a specific definition in VAT legislation, it is reserved for services like welfare, healthcare and education. It might qualify to be zero rated, but depends on what you mean by "art" - digital art will be a digital service, digital services are usually standard rated unless they are zero rated by virtue of them being newspapers or books. If you are a consumer, then I'd expect digital art to be standard rated, if you are a business then I'd expect the sale to be outside the scope of VAT but with the business (you) applying reverse charge to such purchase. If the supplier has charged UK VAT and their invoice shows a UK VAT number, then there is nothing HMRC can do, this is a contractual dispute between customer and supplier. If you believe the supplier is wrong then your only recourse is with the supplier. Details on digital services :-
https://www.gov.uk/guidance/the-vat-rules-if-you-supply-digital-services-to-private-consumers