Scale significant?

The scale of the London bombs is significant. The attack seems to be aimed at disruption rather that a high casualty rate. Unlike Bali and Madrid. The BBC's Security Correspondent, Frank Gardner, has just said similar on BBC1.
So is it for effect - to strike fear - to make a statement to the G8 members meeting in Scotland? Or - and I hope it is not this - is it the prelude to a bigger event later - when people are trapped and the police distracted by the earlier, smaller event?
BBC monitoring has found a website of a group claiming responsibility - it's being checked. Earlier "The BBC's Frank Gardner said Arab sources said the blasts were probably the work of al-Qaeda."
There's a useful reporter log on the BBC News site too.