FAQ21 (my bold):
<<21) What are the residence rules?
A person who is currently not resident in the UK will always be treated as resident in the UK if they spend 183 days or more in the UK in a tax year. If they visit the UK on a regular basis, and spend, on average, 91 days or more in the UK in a tax year (taken over a period of 4 years) they will be treated as resident in the UK. >>