If you arrive in the UK, or leave the UK during the tax year, special rules apply. These are known as ‘split year rules’.
Normally you are tax resident (or not) in the UK for a complete tax year. However, sometimes your residence and work may crossover and the year is split into a UK part and an overseas part.
There are certain rules that apply if you arrive in the UK during the tax year, or if you leave the UK in the tax year. If the special rules apply, you pay UK income tax as a UK resident for income earned in the ‘UK part’ of the year only. Broadly, this means that the non-UK income earned in the overseas part of the year is outside the scope of UK income tax.