EIM70230 | Tax treatment of seafarers: residence status: employment outside United Kingdom territorial waters
From HM Revenue & Customs · Employment Income Manual
A seafarer will normally be regarded as not resident and not ordinarily resident in the UK from the day following departure to the day preceding return where he or she:
has been ordinarily resident in the UK and leaves the UK to take up full time employment on a ship and
the absence from the UK and the period of service includes a complete tax year and
leave spent in the United Kingdom totals less than 183 days in any tax year and averages less than 91 days for each tax year (the average is taken over a period of absence up to a maximum of 4 years).
However, this will not include seafarers whose employment arrangements consist of frequent and regular voyages to and from the UK.