Are you claiming “Split Year Treatment”?
Normally, if you are resident in the UK for any part of a tax year you will be taxed as a UK resident for the whole of the tax year.
However, there are special rules which may apply if you either leave the UK to live or work abroad or come from abroad to live or work in the UK. These special rules split the tax year into a UK part, when you are taxed as a UK resident, and an overseas part, when you are taxed as a non-UK resident.