Why the "complex and coordinated" theory?
This does not have to be a large cell of trained operatives.
Madrid needed that - all the trains were coming IN to Madrid.
WTC needed that for obvious reasons.
London could have been done by one person with 4 plastic shopping bags walking around Kings Cross station before popping outside to catch a bus.
My theory even fits the timing. I dont really want to get into details (though I'm sure somebody somewhere already has).