As an Englishman living abroad, the reserved English character is perhaps more visible to me. The English (and Scots of my acquaintance) just don't get as excited as other nationalities, either about good things or bad things. Do you remember the UK version of Pimp My Ride? You could always tell that the car owner had been primed by the producers to jump up and down and get excited when the new car was revealed -- reactions that always seemed to come very naturally to peope on the US version. But that would be it. The whole exposition of all the amazing features would be greeted by a slight nod and perhaps a bit of a smile. The more wonderful the gimmick, the less reaction there would be.
"And if you press this button, your car turns into a solid gold TARDIS with Shakira at the controls!"
"Hm. Nice."
"Your boot includes a fridge full of food by Heston Blumenthal that is restocked every morning!"
"Yeah, I can see where that would be useful."
Etc.
It might not be "stiff upper lip", exactly, but this is a very English thing.