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For your utter confusion here is HMRC's notice on the matter. You will see that the split year is an extra statutory concession.
RDRM10420 - Residence: Liability to
UK Tax: Individual resident and not
resident in the UK in the same tax year
Strictly, an individual is taxed as a UK resident for the whole of a tax year when they are resident here for any part of it. But, if the individual leaves or comes to the UK part-way through a tax year, the year may, by concession, be split (extra-statutory concession A11).
This means that the UK Income Tax the individual should pay because they are resident here is calculated on the basis of the period they are living here rather than the whole of that tax year. This has the effect of splitting the tax year into resident and not resident periods for the purposes of calculating the tax due.
This split-year treatment will apply to individuals who:
- come to the UK to take up permanent residence or to stay for at least two years, or
- leave the UK to become permanently resident abroad, or
- leave the UK for full-time service under a contract of employment for at least a complete UK tax year and any interim visits to the UK in the period do not amount to 183 days or more in any tax year or an average of 91 days or more in a tax year.
There is a further extra-statutory concession (A78) which also allows split-year treatment if an individual is accompanying their spouse or civil partner when they leave the UK to work full-time abroad, or in the year of return to the UK.
Although the rules for capital gains are significantly different from those for income, there is a split-year concession which relates to the chargeable gains of individuals who come to, or leave, the UK during a tax year (extra-statutory concession D2).
RDRM10135 and RDRM10245 tell you how the liability for the year of departure from, and arrival in, the UK is to be treated.