I think it would need a Cambodia style communist revolution.
OK, WWI goes harder, Britain loses another 50%, (1.5 million dead).
Explosion at a chemical weapons plant kills thousands in 1917.
British Communists follow a neo Luddite agrarian approach, arguing for the destruction of industry and a return to the farming nation there once on the basis that the male population has been devistated and between the deaths from German Zepplin raids and chemical weapons deaths, domestic farming can now support the population.
Mass emigration/deportation to Canada, the US, Australia, New Zealand etc.
Population reduced to about 5 million in the end.