From what I heard from the group claiming responsibility posted on a web site, the bombs were in direct retaliation for Britain's involvement in Iraq and Afghanistan. The G8 summit was just the perfect opportunity to attack and the hallmark of Al-Qaida attack: it was well-coordinated, timed to happen during a major political event, and consisted of multiple attacks on a transportation system at rush hour.
They also said the group has threated to hit Italy and Denmark next. But I'm sure they have many, many targets and will go after them if the opportunity arises.