RFIG21250 | Statutory Residence Test (SRT): Split year treatment: Case 7 - Deemed arrival day
From HM Revenue & Customs · Residence and FIG Regime Manual
The deemed arrival day is the later of:
the date that is the first day of the UK part of the year for the individual’s partner under Case 6
the date on which the individual moves to the UK so that they can live together in the UK with their partner