Brit and avid history fan here! Stiff upper lip is a myth. We used to be a very rebelious lot:

We're taught about Henry VIII, but not about the mass uprising he had to put down (The Pilgrimage of Grace)

We hear about the Battle of Hastings but not the Harrying of the North.

We're never taught about the Enclosure Acts (that stole land from the common folk) and the subsequent uprising and brutal repression (including the Midlands Revolt).

We also had the Peasants Revolt trying to stop the crazy taxation during the 100 year war!

And if we're looking for other acts of rebellion:

The Peterloo Massacre

The General Strike of 1926

The Miner's Strikes of the 80s

The Battle of Cable Street (Police protecting Nazis)

The Battle of Lewisham (Police protecting Nazis)

But it is far, far better for those in power to make us believe we have always been meek and "stiff upper lip"

EDIT: for people looking for a complete list, this ain't it. I just chose a few that were in my mind at the time. I also didn't include anything to do with imposed rule or I'd just gesture vaguely at the island of Ireland.

I also didn't include anything to do with aristocrats fighting each other. This is an incomplete list of popular uprisings to make a point.