Westminster is a part of central London, and the name is used to mean the British Parliament — the group of politicians who make the country's laws — because the Houses of Parliament stand there. So when a news report says "Westminster is worried about the plan" or "the mood in Westminster has changed", it is not talking about the streets or the buildings; it means politicians, their staff, the journalists who follow them, and the whole world of British national politics. This way of using a place name to mean the people who work there is common in news writing: Washington means the US government, Hollywood means the American film business, Westminster means British politics. British people also use it to mark a difference: "Westminster" is the London parliament, while Scotland has Holyrood, Wales has the Senedd, and Northern Ireland has Stormont. So "a Westminster decision" can quietly mean "decided in London, not here", and depending on who is speaking, that can sound a little cold or far away. The word is common in newspapers, on TV news, and in serious talk, but not really in everyday chat. Note that Westminster Abbey is a famous church in the same area — a different thing, though the names look the same, and that church is where kings and queens are crowned. There is also a Westminster in some other countries (a district in London, a city in Colorado, a school), but the political meaning is the one you meet most in reading.