Not if cells were on alert and ready to go as soon as the announcement was made. You may be right, but it seems that the day after the Olympics announcement is an ideal time to strike the city that gets the Olympics.
It was also "the day after the G8".
It was also "3 days after the Fourth of July".
Pretty meaningless.
Far more likely that it had some fool anniversary of some 7/7 historical event - but who cares. On that basis, every day is the anniversary of something.
If they had hit London some days BEFORE the Olympic committee announcement, as did the Madrid bombings, I would agree it had some correlation. It is more likely they erroneously thought that the announcement was supposed to be made on 7/8 than that they decided to hit the Olympic site the day after the announcement.