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HMRC internal manual

Residence, Domicile and Remittance Basis Manual

Residence: The SRT: Country tie

The individual will have a country tie for a tax year if the UK is the country in which they were present at midnight for the greatest number of days in that tax year.

If the greatest number of days the individual  was present in a country at midnight is the same for 2 or more countries in a tax year, and 1 of those countries is the UK, then the individual will have a country tie for that tax year. For the purposes of this SRT test presence at midnight in any state, territory or canton into which a country is subdivided is regarded as presence at midnight in that country.