Resignation from the House of Commons of the United Kingdom
As a constitutional convention, members of Parliament (MPs) sitting in the House of Commons of the United Kingdom are not formally permitted to resign their seats. To circumvent this prohibition, MPs who wish to step down are instead appointed to an "office of profit under the Crown"; by law, such an appointment disqualifies them from sitting in the House of Commons.
Source: Wikipedia — Resignation from the House of Commons of the United Kingdom (CC BY-SA 4.0)