The terrorists were quite competent and all of their devices should have worked as designed.
However, the cellphone service didn't support the mission.
There can now be no doubt that the only way you can get good cellphone service in Central London is to be inside or very nearly inside a building. This is a sign that the Brits have installed devices throughout the area that suppress cellphone signals from the Cell to the handset.
Their airports seem to be equally well-guarded with this new technology.
I have been thinking the same thing. That's probably why the poor ignorant, helpless, little freedom fighters set the car on fire at Glasgow, and didn't rely on a cell phone.