When Spike and Isla go into the gas filled petrol station, the gas is shown as floating above them. But Benzene gas is heavier than air so they would have been wading through it as they walked. So they would not have been able to escape unscathed by ducking down when the soldier sets off the gas explosion.
Spike and Isla are inside a gas station which is filled with explosive gas, a soldier tells them to get down and shoots into the gas to make it explode, and after that Isla and Spike are totally fine not even their hair was burnt. That's not how physics work, not only their hair would burn away, but they would also have had burnt skin.
At the middle of the movie, an "Alpha" rips out a person's head along with his spine, that's just impossible, the vertebrae are not connected to each other that strongly to tolerate that amount of force. Only the head would be ripped along with maybe one or two vertebrae.
During the train scene, at the moment of the attack, Spike yells at his mother, telling her to "get off the train". Given that he has always lived in a secluded community, away from the rest of the world until now, he can't possibly know what a train is, let alone what it's called. However, he might have learned that during a school lesson.
While walking through the mainland, it is shown as being well managed. They walk through areas that have clearly been used as crops or for grazing relatively recently. Long shots show fields with well defined hedges. But no organised farming would have taken place for the 28 years. The land would have become heavily overgrown by the time the film is set.
At the end of the movie, there is a scene filmed in Cheddar Gorge, Somerset where the final Infected attack occurs. A tarmac covered road snakes through the Gorge and it is remarkably clean and unaffected by weeds, grass or other plants which would have taken over road if left unattended for 28 years.
At a point near the beginning of the film Spike picks up a power rangers action figure of the red operation overdrive ranger. This show was released in 2007 and the rage apocalypse happened in 2002.
Spike and Isla see the Angel of the North sculpture shortly after leaving Holy Island. The sculpture is over 60 miles away in Gateshead, much farther than they are shown to have traveled.
In one scene wind turbines can be seen in the background, inactive. These turbines would not have been there 28 years ago when the infection first hits, so they wouldn't have been installed after either.
There is no reason why Continental Europe wouldn't support the British islands by dropping goods from air.
The whole concept of the community living on the island relying on protection from the infected because of the water that separates them from the mainland is not realistic. This is because the causeway foot path that connects the mainland to the island is visible and accessible at low tide twice in a 24 hour period each day and it would be a fact that in those 28 years the infected would have made their way to the coastline at a time during the day with a causeway path is visible and accessible and would have made their way, most likely in large groups, to the island and besieging and overwhelming its defenses.
Kelson was able to use drugs such as morphine to sedate the Alpha long enough to distract or escape from the infected. If anaesthetics sedated them, then there's no reason larger (over)doses wouldn't also have killed them. Given the danger the Alpha posed, it's curious why Kelson let him live.