Trevor Wilkes, Principal of Trewtax Services responded as follows:
Split Year Treatment is not an entitlement, it is something that you have to qualify for. If you don’t qualify then you will be regarded as tax resident for the year of arrival i.e. from 6th April and there taxable on worldwide income and capital gains from that date. It is something to be carefully reviewed and then documented in the Self-Assessment tax return for the year of arrival.
If you qualify, liability to worldwide income and certain capital gains applies from the arrival date. UK income remains taxable come what may.