LONDON — Everyone says that Britain isn’t supposed to work like this, that the chaos of chewing through six prime ministers in a decade is just “not normal” for one of the most stable and ancient democracies in the world.  But what if it is?  As Andy Burnham prepares in all likelihood to become the seventh person in 10 years tasked with forming a government, he stands to inherit a Labour party full of inexperienced MPs who have — sometimes literally — grown up in the era of Brexit politics.  Of