As if you needed reminding ...
The United Kingdom was a member of the European Union (formerly known as the European Economic Union, and in English–speaking countries as the European Common Market) from 1973 to 2020. It was never in the Eurozone, having negotiated an opt–out as part of the Maastricht Treaty (concluded in 1992).
The UK formally left the EU at 23:00 GMT (midnight CET) on 31 January 2020. It continued to participate in the European Union Customs Union and the European Single Market during a transition period that ended at the same time (23:00 GMT, midnight CET) on 31 December 2020.
© Haydn Thompson 2021