What elections are there in the UK?

These elect all 650 MPs who make up the House of Commons.
The Fixed-term Parliaments Act 2011 introduced fixed 5-year terms for governments. Before 2011, the prime minister could call a general election at a time of his or her choosing within their 5-year term.
Prime ministers must now seek a two thirds majority in the Commons if they wish to trigger an early general election – April 2017.

Local councillors are elected for fixed 4-year terms.
In London, there is an elected mayor and assembly, e.g. Sadiq Khan elected in 2016.
Police and crime commissioners (PCCs) are also elected in England and Wales.
The 2012 Police and Crime Commissioner elections had record low turnout, with an average of just 15%, undermining the notion of elected and accountable police officials.

A by-election is held to choose a new representative if a constituency seat (geographical area that elects one or more representatives to a legislative assembly) in the House of Commons, devolved assembly or English local authority becomes vacant because of the death or resignation of an elected member.
MP Zac Goldsmith promised he would not stand as a Conservative candidate at the next election of the party backed the Heathrow expansion, meaning he stood as an Independent in a by-election in 2016.