Since it happened exactly 2 weeks after the first bombings, at almost the exact same time of day, with exactly the same targets, I would think it's some ****er trying to piggyback on the emotional effect of the first bombings, because that's all they can do - they can't even properly kill people, which is a somewhat crucial component of being a terrorist.
"Hey Joe, does damn Londoners sure got screwed. Let's build ourselves a bomb and do the same"
The sheer fact that the date and implementation were significant distinguished these bombings from previous ones (9/11, Madrid, Bali, Nairobi embassy etc) which happened on non-significant dates. That is to say, these were for image while the real ones were for practical utility.