Easy. After losing WWI, with status quo ante bellum between Germany and the UKand severe conditions imposed upon France, Serbia, and Russia, the UK's economy goes into a tailspin. With general strikes becoming common in the war, but being broken up due to war conditions, the labour movement has a lot of practice. This practice leads to the biggest general strike in the history of Britain. This strike is much harder to crush, since the public opinion is more sympathetic towards labour due to there being no war on. Furthermore, the government and Crown are very discredited by involving themselves in a Continental affair and killing millions of Britain's boys.
The attempted crushing of the strikes fails and leads to civil war. The establishment of a socialist republic in Britain ensues and the Scottish Parliament is dominated by the Scottish Socialist Party, one of the few major parties legal in the Worker's Commonwealth.