Battle of Hastings

Considered the most formative event in contemporary British history, the invasion of the Normans in 1066 was really just the latest band of aggressors to subjugate the English. The Corporation of London did a deal with them and the City of London was spared from the doctrine of their new overlords. The shared language of Latin created a 'common' platform between the guilds of the Corporation and the Catholic Normans. This is where the supposed centre of British democracy, the House of Commons, gets it's name.  

The so called Normans were the descendants of the Vikings who sailed up the river Seine and subjugated the French. After approximately a century of enjoying fine wines, fine cheeses and femme fatales, they decided to unseat the squabbling Saxon fiefdoms of Brittania. Since then, the south east of England has become synonymous with a population who never say what they mean. Their dysfunctional social ways have contaminated the rest of the British Isles, the former colonies and through the spread of the English language, pretty much the entirety of planet Earth.

Note: Actual use of English may also extend further