If you're going to go to the trouble of making and planting a bomb, you're going to want the big bang. And what would disrupt the city more--some dud bombs or massive death and destruction?
Maybe all (or almost all) acetone in UK (and in US, too) should henceforth be spiked by trace amounts of heavy metals (like, say, cobalt, iron, or manganese). These would normally cause accelerated decomposition of any peroxides around - and make any acetone peroxide prepared from such a material particularly unstable, more than it normally is.
'...It could be bad workmanship or it could be a deliberate attempt to disrupt the city two weeks later."
What do you expect, military explosives experts? These are teenagers who are dumb enough to believe The Big Lie from their fascists coaches.
(The Big Lie = tell a lie often enough and eventually it becomes true)
I remember an MSNBC interview of a suicide bomber- he was partially retarted and the only reason he didn't go through with it is because a different terrorist organization recruited him a couple days before he was going to blow himself up.
Perhaps it is just an in our face brag ... see we can get to you anytime we want. Making the point that all the tighter security could be penetrated at will.
Thus the two weeks to the day as well as similiar hour of day and the exact same number ... three trains and a bus just as it was two weeks age...
Makes Londoners nervous and unsettles some.