So you think the PM has been told it a terrorist attack on quess work?
It is a terrorist attack because the intent was led by ideology, aiming to induce terror in the name of an idealistic crusade against our culture. It doesn't mean that the guy isn't a nutter - all it means is that the ideology was the excuse used to commit the act of violence. Ideology or not, these people would have a predisposition to violence.
I'll give another direct example - British woman decapitated by a Bulgarian man in Tenerife.
http://www.guardian.co.uk/world/2013/feb/18/jennifer-mills-westley-murder-trial
He said he was an "angel of Jesus Christ who is going to create a new Jerusalem".
Now, if that was a Muslim he killed, would you understand Muslims labelling all Christians as murderous loons on the back of one instance of extreme violence from one individual lunatic?