For example, person from England is English, and Scotland is Scottish.
If I only know the person is from UK, how can refer to this person?
What is making me confused now is if a person from Northern Ireland can also be called "British"?
As many people in Northern Ireland are Republicans, who would like to see Northern Ireland independent of Britain, I would not use British.
A person in Northern Ireland is British by citizenship regardless of their political stance.
Some might consider themselves British only, some British and Irish and others Irish.
Some call themselves Ulstermen.
It may help to keep in mind that the name of the country is "the United Kingdom of Great Britain and Northern Island." This is of course too much of a mouthful to use in informal speech or writing, so most persons refer to the country as "the UK" or "Great Britain."
Citizens of the UK or Great Britain are called "British subjects." Hence, the correct adjective is "British." (Accepted slang terms: Brits, the Brits, a Brit.)
Some argue that the UK is an amalgam of separate units: Wales, England, Scotland, and Northern Ireland; and it is better to speak of the Welch, the English, the Scots (a Scot, adj. Scottish, but never scotch, a type of whiskey), and the Irish. However, that idea leaves us without a non-specific term for citizens of the UK.
I'm learning new things on this forum everyday.
Great site. Thank you.
Actually, on British passports today we are billed as 'British citizens', the EU having deemed 'British subject', i.e. subject of Her Majesty the Queen, to be a bit medieval. Technically, if I'm not mistaken, British sovereignty still lies not in the people but where it has long lain: in Parliament (or, more technically still, in the Queen in Parliament, or whatever the anachronistic expression is).
As to N. Ireland, where I happen to come from, I suppose everyone's technically a British citizen regardless of their politics, but as Tdol points out, it's a dodgy issue (let's not start on Scottish or Welsh nationalism, either). Luckily for N. Ireland Nationalists who wish not to have to travel on a British passport, the Republic of Ireland has long awarded Irish nationality to anyone who could prove they had a parent or grandparent born in pre-Partition Ireland (i.e. before 1922), or to anyone who had a parent who was already an Irish citizen - I believe they've tightened the rules a little recently, but you get the idea. So, a fair whack of N. Ireland's population today, including myself, are still entitled to an Irish passport should we wish, and presumably the Nationalists among us have been exercised their entitlement for years.
Under the Good Friday Agreement of 1998, a citizen from the north of Ireland can be classed and class themselves as British or Irish, or both.
It is a political anomaly that people insist on classing us as British as the Act of Union of 1800 states quite clearly that Britain and Ireland are different:
they are hereby declared to be, the articles of the union of Great Britain and Ireland, and the same shall be in force and have effect for ever, from the first day of January, which shall be in the year of our Lord, one thousand eight hundred and one: provided that before that period an act shall have been passed by the parliament of Great Britain for carrying into effect, in the like manner, the said foregoing recited articles.