http://en.wikipedia.org/wiki/Bloody_Sunday_%281972%29
[Yes, it was a long time ago, but I don't think anything about our society here in the UK has changed to such an extent that would make this fundamentally impossible.]
It was an unfortunate coincidence at a point in time of aggressive soldiers, civil disobedience, no-go-areas and the presence of armed PIRA units.
For the next 30 years of 'troubles' there was never an action like that again by regular army forces, despite constant provocation and attacks against them.
The SWAT / armed police raids, on the other hand, are occurring every day in the USA.
Actually, on the that subject I'm not particularly worried about a general level of violence by police in the UK. However, I am concerned that we could see something like the Troubles again if we continue to treat immigrant populations as scapegoats for all the ills in our society.
"What happened on 17 October 1961 and in the following days against the peaceful demonstrators, on which no weapons were found, morally forces us to bring our testimony and to alert public opinion... All guilty people must be punished. The punishment must be extended to all of the responsible people, those who give orders, those who feign of letting it happen, whatever their high office may be.."
reminds me of the coroner at the inquest for the Bloody Sunday victims, himself a former Army officer:
"They were shooting innocent people. These people may have been taking part in a march that was banned but that does not justify the troops coming in and firing live rounds indiscriminately. I would say without hesitation that it was sheer, unadulterated murder. It was murder."