VATREG39400 | Registration in respect of relevant supplies: voluntary early registration
From HM Revenue & Customs · VAT Registration
There is no provision within Schedule 3A for a person to register earlier than the date they are required to register.
However, if a person chooses to register from an earlier date, on a voluntary basis, then you should consider such requests in accordance with VATREG21000 which gives guidance on dealing with applications for voluntary registration.