I'm still watching it as I type this, but they've almost lost me because:

1. A lookout sees two girls breaking the quarantine, yet they have time to walk around London, explore a pizza shop, grab a motor bike, drive out to the country and play around for quite a while before they're apprehended.
2. A survivor is in medical quarantine and a bureaucrat from the city has an all access pass that lets him open every door in a highly guarded facility. No one notices him, no one is guarding the patient, and he has time to have a talk, become infected, kill, go on a blood splattering rampage in the room, and still no one has noticed.
   Lame.

Dude, the second a person without symptoms (the wife) somehow managed to carry and spread the virus, all bets were off.
The soldiers knew (to their knowledge) no infected had come into the safe zone, and still, somehow the infection took hold.
So anyone with as much as an infected bloodstain on them was fair game.

Or why the civilian containment room was in the beginning of the initial outbreak was so pathetic.

1. The soldiers were ordered to shoot on sight any moving targets, even if it it means the target is a healthy person.
2. Probably got taken out.