All 650 members of the UK Parliament are elected using the first-past-the-post voting system in single member constituencies across the whole of the United Kingdom, where each constituency has its own single representative.
All MP positions become simultaneously vacant for elections held on a five-year cycle. The Fixed-term Parliaments Act 2011 sets out that ordinary general elections are held on the first Thursday in May, every five years. However, with approval from Parliament, both the 2017 and 2019 general elections were held earlier than the schedule set by the Act.
If a vacancy arises at another time, due to death or resignation, then a constituency vacancy may be filled by a by-election. Under the Representation of the People Act 1981 any MP sentenced to over a year in jail automatically vacates their seat. For certain types of lesser acts of wrongdoing the Recall of MPs Act 2015 mandates that a recall petition is opened; if signed by more than 10% of registered voters within the constituency the seat is vacated.
To be eligible to stand as an MP a person must be at least 18 years old and be a citizen of the UK, a Commonwealth nation, or Ireland. A person is not required to be registered to vote, nor are there any restrictions regarding where a candidate is resident.
The House of Commons Disqualification Act 1975 outlaws the holders of various positions from being MPs. These include civil servants, police officers, members of the armed forces, and judges. Members of the House of Lords are not permitted to hold Commons seats. Members of legislatures outside of the Commonwealth are excluded, with the exemption of the Irish legislature.
The first duty of a member of Parliament is to do what he thinks in his faithful and disinterested judgement is right and necessary for the honour and safety of Great Britain. His second duty is to his constituents, of whom he is the representative but not the delegate. Burke's famous declaration on this subject is well known. It is only in the third place that his duty to party organization or programme takes rank. All these three loyalties should be observed, but there is no doubt of the order in which they stand under any healthy manifestation of democracy.
Theoretically, contemporary MPs are considered to have two duties, or three if they belong to a political party. Their primary responsibility is to act in the national interest. They must also act in the interests of their constituents where this does not override their primary responsibility. Finally, if they belong to a political party, they may act in the interests of that party, subordinate to the other two responsibilities.