Reply #10: it was probably the icing on the cake, for them

[Lisa]

Planning the attack for during the week of the G8 meeting was likely deliberate, not just for the political impact (and media coverage, with so much coverage of the UK) -- but also for practical reasons. Security would have been more focused on protecting the visiting leaders at the other end of the country, so they may have thought it would make things easier.

I suppose it's possible that they had originally planned it for Friday but moved it up a day after the Olympics news, just out of spite (and to take advantage of the extra international press coverage of London).