The G8 is happening way up in a remote place in Scotland, not in London. What may have happened is that the G8 acted as a distraction for the security people.
I doubt it made any real difference though, this sort of attack is almost impossible to halt directly. Past experience with the IRA suggests that you need to get the cells before they're active, not after they've gone active.
Which is what our Patriot Act is designed to do.