Your challenge is to have GB descend into civil war with any POD before 1930 and after 1800.
The Free State emerges, the Irish civil war begins, and the anti-treaty forces win. The Irish army then invades Northern Ireland, but the British Government is reluctant to join the fray, concerned that Italy and Spain, and possibly Germany will in turn declare war on them and start a second World War. Carsons UVF take up the fight by themselves, despite being much weakened by casualties in ww1. Huge numbers of Irish emigrants from the UK and USA volunteer for the Irish cause, whilst British protestants, especially from Scotland, go to the aid of the loyalists. Some ideological factions join the battle too, some leftists support the Irish nationalist cause, while others (mostly from the CPGB) support the Union. With O'Duffy's Blueshirts now fighting for the anti-treaty forces, Oswald Mosley (an admirer of Michael Collins) leads a small detachment of volunteers to join what they see as their compatriots. Others, wearing the very same 'iconic' black shirts, go to volunteer for the UVF. Some leftists from the International Brigades also rally to the Ulster cause, seemingly because of Franco and Mussolinis speeches in support of Irish Catholics. Because of the great number of British citizens (both from Northern Ireland and the mainland) and the majority of the fighting occuring within de facto British territory, the conflict is considered to be a British civil war, although in Ireland its known as the second civil war in some sources.