Our mutual friends at HM Revenue and Customs have issued guidelines on record keeping.
The length of time they will expect you to keep records varies, depending on the type of tax involved. Since Customs declarations involve more than one type of tax, this can get confusing and difficult to manage. We therefore recommend you do as we do and keep your records for the longest prescribed time – that is published as six years. However, what they really mean is current year plus six years. If you keep your files for seven years, you can’t go wrong, in theory.
Please be aware that Customs can raise financial penalties against you if you fail to maintain adequate records. They are nice like that.
P.S. EORI reminder – you must have an EORI post Brexit. Start this process at: https://www.gov.uk/eori