VATREG02150 | Basic principles of registration: meaning of taxable person
From HM Revenue & Customs · VAT Registration
The VAT Act 1994, section 3(1) provides that a taxable person is someone who is, or is required to be, VAT registered.
The VAT Taxable Person Manual contains guidance on how to determine who is making a supply and whether the supplier is a taxable person (External users can access the manual here).