EIM42804 - Meaning of domicile: claim to be not domiciled
IM1630 tells you about what is meant by domicile.
See EIM42806 for the action to take when an individual, who is only liable to tax on employment income, claims to be domiciled outside the United Kingdom.
See IM1635, if there is some other liability, for example, on Miscellaneous Income under the special rules for Foreign Income in Part 8 IT(TOI)A 2005 (previously under Case IV or V of Schedule D).