By-elections in the context of "Elections in the United Kingdom"

There are five types of elections in the United Kingdom: elections to the House of Commons of the United Kingdom (commonly called 'general elections' when all seats are contested), elections to devolved parliaments and assemblies, local elections, mayoral elections, and Police and Crime Commissioner elections. Within each of those categories, there may also be by-elections. Elections are held on Election Day, which is conventionally a Thursday, and under the provisions of the Dissolution and Calling of Parliament Act 2022 the timing of general elections can be held at the discretion of the prime minister during any five-year period. All other types of elections are held after fixed periods, though early elections to the devolved assemblies and parliaments can occur in certain situations. The five electoral systems used are: the single member plurality system (first-past-the-post), the multi-member plurality, the single transferable vote, the additional member system, and the supplementary vote.

Elections are administered locally: in each lower-tier local authority, the polling procedure is operated by the returning officer or acting returning officer, and the compiling and maintenance of the electoral roll by the electoral registration officer (except in Northern Ireland, where the Electoral Office for Northern Ireland assumes both responsibilities). The Electoral Commission sets standards for and issues guidelines to returning officers and electoral registration officers, and is responsible for nationwide electoral administration (such as the registration of political parties and directing the administration of national referendums).