What is the standard for triggering a vote of No Confidence?

An important point: the vote of no confidence is not a vote to remove the Head of Government. In the Westminster system, the cabinet has collective responsibility. That means all cabinet ministers are co-equal. A confidence vote is a vote in confidence of the whole government, that is the Prime Minister, the cabinet, the junior ministers and all the non-civil-service government positions, as a whole. It is an all-or-nothing proposition. They stand together, they fall together. There is no requirement for a motion of no confidence to be carried beyond a simple majority vote, and there is no requirement to meet any specific threshold of misbehaviour to trigger one. A motion of no confidence can be brought at any time and for any reason, subject to the procedural rules governing votes in Parliament. Because the procedural rules for parliament allocate certain days on a regular basis for opposition motions, if the opposition wishes to bring such a vote, they have plenty of opportunity to do so, albeit at the cost of the opportunity to debate some other topic, so doing so frivolously is itself seen as a waste of time. In principle, there is no reason the government itself could not bring a motion of no confidence, and during the era of the Fixed Term Parliament Act this was discussed as a way to circumvent that legislation, and indeed a major reason for scrapping it. In addition to an explicit and direct motion of no confidence, there are two votes per year that are automatically treated as inherently confidence motions, that is if the vote fails, that has the same effect as a motion of no confidence passing. One is the King's Speech and the other is the budget. The King's Speech sounds fancy, but in reality it is simply the speech prepared by the government, read out by the King at the beginning of the session (ie that parliamentary year), outlining the legislation the government intends to bring forward. In essence a "this is our plan for the year". The Budget is pretty straightforward. No budget = no confidence. No to the King's Speech = no confidence. In the event such a vote is passed, it is not an instant general election: other groups within Parliament, such as the opposition, can make an attempt to form a government that can survive a confidence vote. It would take some unusual but not impossible circumstances for that to happen (look up the King-Byng affair in Canada for an example of this sort of thing). If no government can command the confidence of the electorate, then the King dissolves parliament and issues a writ for a general election.