On 9 August, hours before the arrests, the UK Home Secretary John Reid gave a major speech to Demos (a UK think-tank) hinting at a new round of anti-terror legislation and claiming that the country was facing "probably the most sustained period of severe threat since the end of the second world war". The following day he broke the news, the Prime Minister being abroad on holiday.
Wait a second. A major victory against this incredible and constant threat and no one can be bothered to haul a podium to the beach so Tony Blair can make this announcement himself? Why is this clearly important development too mundane for a out oworld leader to dry himself off, put on a suit for two hours and make a simple speech? It's not like he was giving a press conference about embaressing sexual exploits; that I can understand wanting to skip.
Seriously, this kind of stuff is political gold. Why did Blair opt out? The first thought is that he wants to give someone else the limelight, seeing how he's not running for a fourth term. The second thought is scarier: by having the same person who called this time an era of great and sustained danger in need of more laws announce these major arrests so soon afterward, it makes the legislation seem all the more urgent and his calls all the more justified.
In the US, we see the National Guard being mobilized, air marshalls being sent to the UK and the banning of all liquids on US flights, all to prevent something that's already been prevented. I'm no expert about police investigations in England but I doubt requiring passengers to take off their shoes or turn in their gatorade helped London Metropolitan Police foil this plot.