The sign of four

They operate in groups of four. Large explosives that require rucksacks for concealment. To ensure the success of their mission, it helps if the location is one where the presence of such bags will cause least alarm. A public area, with lots of people. They want a blast, and a location, that will imprint itself on memory, that will affect people's lives, that will make everyone sit up and take notice, and fear.

Prediction: The four primary airports of London. Heathrow. Gatwick. Luton. Stansted.

Analysis: A young man arriving at an airport with a backpack. No problems in getting there, no problems standing out once you reach there. You do not have to go through any security clearance to get into the airport - only once you have checked in and go into the departures area. There can often be huge queues and crowds of people waiting to check in. Completely open area, lots of people area about, nothing to restrict the force of any blast. Just wander in with the backpack and join a queue. The alternative would be the arrivals hall. Less crowds, but perhaps less security too.

Disclaimer: The above is pure speculation. We don't know any terrorists, and we do not advocate the course of action described above.