www.gov.uk/government/publications/sailing-a-leisure-craft-to-and-from-the-uk latest guidance. Registration is of little relevance. Conditions for entry of the boat are determined by its VAT "status". If it does not have UK status and is owned by a non resident it can enter for up to 18 months. If it does have UK status, irrespective of ownership or flag there are no restrictions. In any case reporting as per the guidance is essential.
