The history of these islands is more complicated than some of the postings here which seem to boil down to “The English invaded everyone else and are racist”.
Let’s look at what actually happened (even this is a nutshell)
1603 the King of Scotland becomes King of England because the Queen of England dies without having any kids.
1707 Scotland and England become one state. The Parliament is based in London because this is the biggest city in the islands. Scots send MPs to the British Parliament.
NB the Battle of Culloden which is frequently misrepresented as an England v Scotland battle concerned whether a Catholic or a Protestant should be the rightful King. Both contenders were descended from the Stuarts, who were Scottish.
1801 Ireland, which was a self governing colony with its own Parliament becomes part of the UK. The Irish are rightfully pissed off because Catholic’s (90% of the population) are disenfranchised. In 1829 Catholic emancipation means that Catholics can now vote and stand for Irish seats in the UK Parliament. For nearly a century Ireland is part of the UK.
In the late 19th century devolution along the lines of what we now have in Scotland is proposed. The legislation fails. It is passed in 1914 but then WW1 gets in the way of it actually happening.
Meanwhile, rather than using the ballot box and waiting to see if devolution works after the War, in 1916 the IRA have an uprising and start a civil war that results in Ireland being partitioned because the Protestants in the North don’t want to be governed by a Catholic theocracy in a United Ireland (see most Irish social policy up to the 1990s).
Northern Ireland itself was self governing but there was an in built Protestant majority in the NI Parliament resulting in policies that ignored or repressed Catholics.
This results in unrest so the British government imposes direct rule in the 70s and sends soldiers to keep the peace thinking this will be fairer to Catholics but is perceived as an invasion by Catholics, resulting in the troubles.
The Union Flag has been used by all sorts of groups over the years- Loyalist Paramilitaries, Football Hooligans, Fascist groups and the like, but it’s easy for anyone to adopt a flag or a symbol and misuse it.
The country to which the flag belongs was one of the first developed societies to abolish slavery, one of the first to pass legislation to end discrimination and protect minorities, and has a colonial era history which was a mixed bag but in many ways far more benevolent than other European colonial powers. For instance in African legislative assemblies the vote was never based on race but property. While this discriminated against the poor, and certainly discriminated indirectly it still meant that a well off African could vote when a poor white settler couldn’t.
It was an attempt to “civilise” Africa and exploit natural resources at the same time. Not a good thing overall but clearly not all bad given so many African countries choose to remain members of the Commonwealth as independent countries. Meanwhile the French and Belgians basically treated their African colonies as slave labour camps.
Even the above is just a potted history but far more complicated than English = bad, everyone else = victim.