England has never been a republic.
Republicanism has long been a force in British politics and the picture depicts the British Republican Flag, originated in 1816, in use until at least 1935. Moreover, a republican government was briefly established in the mid-17th century, after the Parliamentarian victory in the English Civil War. The countries that now make up the United Kingdom, together with the present Republic of Ireland, were briefly ruled as a republic in the 17th century, first under the Commonwealth consisting of the Rump Parliament and the Council of State (1649–53) and then under the Protectorate of Oliver Cromwell (1653–58). The Commonwealth Parliament represented itself as a Republic on the classical model, with John Milton writing Latin justifications for use as propaganda in Continental Europe.