The plot was very well along - if not as far along as UK domestic security forces and the CPP preferred for ensuring guilty verdicts. But do not confuse not being as far along as the lawyers preferred with the plot not being viable and in action, let alone "imminent;" the problem of not having tickets could have been overcome in an hour or a day as specific flights had been selected/targeted. In fact, all of the bomb-makers and most of the conspirators admitted to the essence of the plot (if not admitting specifically to attack aircraft).

And the plan was viable; read this article by James Lewis, a former EOD officer: https://www.theregister.co.uk/2008/0...bomb_verdicts/