In reply to violentViolet:
There is also the english law concept of "domicile" - which is a rather old concept and different to citizenship/nationality or simple residence. You see it most talked about in terms of people abusing the rather advantageous tax rules in the UK which apply to individuals who are tax resident here (a much shorter term concept) but are domiciled abroad.
Domicile is basically where you see your true home. So, if you are essentially British but go to, say, the USA and live and work there for many years, but still think of yourself as a Brit, then you are domiciled here in law (even though you may not have stepped foot in the UK for decades).
Everyone is born with a "domicile of origin". Usually this is where you are born but, in fact, it's the domicile of your father when you are born. So, if your father were an immigrant to the UK (from, say, Poland - being topical), you would be born with a Polish domicile. And that could stick with you even if you live in the UK for many years, potentially for life if you still think of yourself as Polish. It's irrelevant that Poland may not have the same of similar concept from an English law perspective.
Ignoring some minor complexities, once you are an adult, you can only change your domicile by clearly abandoning your domicile of origin and taking on a "domicile of choice" elsewhere. Typically you will look at where the individual's social and economic ties are strongest. The onus of proof is on the person claiming the change in domicile. This means that it's hard (from a tax planning perspective) to show you have lost your UK domicile when you emigrate - so even though you become non resident you may remain UK domiciled for always. Conversely, if you are coming to the UK, it's very hard for the Revenue to show that you have lost your domicile of origin (eg Poland) and have taken on a domicile of choice in the UK.
All very interesting
I suppose my point was that I believe we rarely use domicile in the UK now except in tax planning. Maybe we should use it a bit more. It's a little like the infamous "cricket test" but rather more rigorous! You may even acquire a British passport & citizenship and still remain non-UK domiciled (so you can see the subtle difference between the concepts).