This film really is utterly preposterous. Unbelievable in the extreme; plot holes every where. It makes absolutely no sense whatsoever. It still kicks ass though. Hugely enjoyable.
It is set in the future, after a virus struck Glasgow the British govt decided the only way to deal with it was to wall off Scotland. Leave the dying to the dead and make sure no one gets through. And so Scotland is abandoned. But years later the virus makes an appearance in England. So a team is sent north, through the wall to the survivors to see if they can find a cure.