Al Qaeda is planning a major political assassination to head off a series of terrorists attacks (from dgci). I'm speculating that the target is Tony Blair.
There is a lot of opposition to the war in England and even in Blair's own party. Enough that it is reasonable to believe that if they killed Blair, England would be out of the War on Terror. This would be a tremendous victory for al Qaeda, dwarfing what they did in Spain.
By contrast, killing Bush would only piss Americans off. Al Qaeda can't be too dense to realize that. Faltering public support for the war would coalesce instantly behind Cheney.
Italy is another possibility, but it probably isn't any easier to kill Berlusconi than Blair (maybe harder because Political Correctness has gone so far in England and the Islamists have gained so much ground there) and the rewards for killing Blair are enormously higher.
Another clue is that when someone spilled the beans about the fact that Kahn was a double agent, a lot of arrests were made in England but nowhere else I heard of. That suggests that something big was going on in England.