The story is this: Scotland is plagued by a virus and segregated from the rest of the world (i.e. England) by a wall along the lines of Hadrian's. For some time (and some unfathomable reason), this works: the virus is contained for as long as it takes to make a ~seven year old into a beautiful woman (say 20 years), until suddenly it breaks out in some London ghetto, due mainly to the fact that the Prime Minister's Dick Cheney is a Dick Cheney character (although younger and not quite as fat).
Meanwhile, Dick Cheney has gathered information that there are, in fact, survivors up in Scotland, and where there are survivors there must be vaccine, or immunity, from which a vaccine hopefully can be created. At this point, disease cleverly turns into political allegory, since Dick Cheney obviously needs immunity for his crimes.
Our heroine and a team are consequently sent to Scotland to find a doctor character presumed to have a cure or vaccine or whatever. They go to Glasgow, and we notice that despite 20 years with no productivity, and a country left in total disorder, the Scots haven't changed one bit (except for the many who mysteriously lost their dialects): They are still a mean-spirited, violent people of dirty hooligans with a fashion sense stuck in the late 1970s (oh, but they do love each other, two of them). The genius of the film lies in the location to Scotland, as it makes zombies unnecessary: the healthy and living Scots act almost exactly like the zombie creatures infested with the rage virus in 28 Days Later, with as little regard for life, for pain, and for their own health as only Scots can have.
The English intruders are taken captive (one or two are also killed, and two lesser characters stay out of this altogether), and the Scots act out a few scenes from Mad Max 3, until our heroes escape and are once again captivated -- and this time taken to a castle in the countryside, as if to give the audience the experience of both the urban and the rural Scotland. Both suck, and are at war with each other.
The leader of the highlanders or whatever is the very same doctor they went looking for, who then introduces yet another political allegory. The ones living in Scotland live there because natural selection has bestowed upon them the gift of hardship to such a hostile and disease-ridden environment, and England can bugger off. The disease is here taken as Scotland, which only Scots can endure. England certainly needs the Scots. Do the Scots need the English?
The doctor-king thinks not, and has our heroine executed. Except she executes the executioner instead, turning the allegory against him: the English can endure the Scots (she's actually Scottish, but a loyal servant to the Queen, King or whatever), making the English the rightful rulers of the Scots and everything Scottish. Most of the film is about proving not only this, but also showing that a Scotland without loyal subordination to the English is a Scotland deep in chaos.
It does so very convincingly.
As pure entertainment, though, I find it somewhat lacking.