The stiff upper lip is underpinned by understatement, a very British way of speaking, which resolutely refuses to succumb to drama, excitement, or high emotion.
Sometimes British understatement is undeniably humorous: a famous example is the Monty Python sketch where the Grim Reaper turns up at a suburban dinner party and insists that all the guests accompany him. "Well", one of the party guests remarks, "that's cast rather a gloom over the evening hasn't it?"
But understatement isn't always deployed to raise a laugh, it permeates British speech. Conversation is littered with moderating expressions, such as 'quite', 'rather', 'a bit', 'actually'. 'Not bad' is high praise, and 'not bad at all' is positively euphoric.
Nowhere is this more apparent than in talk about the weather: 'a bit nippy' is considered an appropriate description of sub-zero temperatures, 'rather damp' describes a monsoonal downpour.
Don't confuse understatement with under-reaction - read between the lines and you'll find the missing drama and emotion.