Britain is a very arcane and quite anarchranistic a country. It in many ways resembles Austria-Hubgary more than any modern country, in the sense that is a union of different peoples, or to put it bluntly, an English empire. It also reassembles in many ways a pre-democratic, pre-enlightenment empire with all 'ye olde' institutions such as the nobility and city guilds. Also, lack of a written constitution.
The only part of the United Kingdom to follow the trend was Ireland after the First World War, and it is now a modern, secular, tricolour, democratic Republic.
Have Britain, around the time of the First World War, follow the AHE, the Ottomans and the German Empire, along with Tsarist Russia, to the knackers yards, with all four countries becoming independent and sovereign nations (England doesn't have to become a republic, but the three others do.) They all have to become modern nation states, with ye olde institutions and traditions done away with, and with written constitutions.
Bonus points if England gets to inherit her former states' colonies as though nothing happened.