The British in general are fairly phlegmatic in the face of hardship. Stiff upper lip may well be a cliche, but its a cliche rooted in reality.
And frankly its hardly the worst or most disruptive Terrorist attack on London, or Britain as a whole, and pales into insignificance compared to the Blitz of WWII, when this sort of death and injury total would be a daily occurence for months on end.
There have been more fatalities in accidents on the underground system, such as Moorgate and the Kings Cross fire than in this attack.
Frankly, compared to death tolls in Madrid and other targets of Terrorism since 9/11, we seem to have gotton off relatively lightly. Sure, it is shocking and tragic to those involved directly and the friends and family, and my sypathies go out to them. But in the morning it will be business as usual (or as near as can be with a few tube routes closed until the crime scenes investigations are completed, which will probably be done over the weekend)
Sure, security will be tightened up a bit, and police will be working to arrest those involved or their accomplices, but for the average Londoner it will all just be an anecdote for dinner parties/pub chats etc in a couple of weeks time.