I don't think this necessarily has anything to DO with G8 itself, I think it was more an attack of opportunity. A fair bit of the UK's security forces were centered about Perthshire, Gleneagles due to G8, leaving London uncharacteristically open for attack.

However, regardless of the currently nitpicky details of who and why, my thoughts and prayers go out to each and every one of you who are suffering in any way as a result of this.