Domicile is a general law concept and is distinct from nationality or residence. It is not possible to list all the factors that affect your domicile but broadly speaking, you are domiciled in the country where you have your permanent home.
A UK 'resident' for tax purposes is anyone, of any nationality, who spends more than half the year (strictly 183 days) in the UK for any reason - work is a common one. By itself that would normally make them liable for UK income tax on their worldwide income. The anomaly, however, is that the same individual could simultaneously be 'non-UK domiciled' because their 'permanent legal home', as the tax rules put it, is not the UK. Non-domicile status is given to people who were either not born in the UK or whose parents spent most of their lives in another country.
Even a British citizen who has spent most of their adult years outside the UK, could still be liable to pay UK inheritance tax. Probably the most famous case of this sort involved the actor Richard Burton. Although he lived in the US for 27 years, because his body was buried in Wales, the Inland Revenue deemed him still residing in the UK and took £2.4m in inheritance tax (IHT).
If you would like to understand more about UK inheritance tax (IHT) clickhere.
There is no way to conclusively determine if you have lost your UK domicile status, but the HMRC does provide a document (DOM1) whereby you can declare yourself a non- domicile. Once complete, simply send to your last known tax office in the UK. If you are not sure where that is, you can give them a call on +44 151 210 2222. If you would like to read a more thorough overview of UK domicile law pleaseclick here.