Opportunistic attack timing?

Perhaps it was perceived that the G8 summit would draw spare security resources away from London's infrastructure, thus providing an opportunity for attack.

It is still too early to say whether suicide attacks were involved or not, but it's worth keeping in mind that premature detonation in the past has taken out would-be bombers in the UK. In 1996, a member of the IRA died from his own bomb as it exploded while he was riding a bus. He was certainly no suicide bomber.

Also, there were no suicide bombers in the Madrid attack. Although three suspects apparently blew themselves up when cornered afterward, they were not in a hurry to die otherwise, as one suspect escaped the explosion.