For UK tax purposes, domicile is important for determining how you are taxed on your worldwide income. Domicile is defined where a person has their long-term, permanent home. It is different from citizenship or residence.
Your domicile or origin is the same domicile as your father’s domicile at the time of birth.  If your father changed domicile while you were still a dependent, your domicile will also have changed. You can, however, change your domicile. In order to do so, you must cut links with your previous domicile, move to a new jurisdiction and have a permanent home in that jurisdiction.  It is difficult to acquire domicile of choice compared to domicile of origin, and the responsibility to prove that your domicile has changed lies on you.
Most expats in the UK are considered non-UK domiciled.