I could see how the remaining population of Britain would pressure their government to return earlier than might be safe. However, the security in 28 Weeks Later is so wildly incompetent that it ruined the movie for me.
First problem is containing the people. There are three ways off: air, sea, Channel Tunnel. The Chunnel is easily dealt with, seal it off.
People escaping on aircraft are a problem. They would have to be detected and intercepted which means radar coverage from every angle. The existing civilian radar network does not provide complete coverage and can be evaded. You'd need to ring the island with military grade radar. A combination of ships at sea and coastal installations at all possible coastlines in range: which is most of the Western Hemisphere. Administratively, no aircraft which cannot prove its point of origin would be allowed to land. Aircraft from Britain would have to be diverted to airports with quarantine facilities, or simply shot down.
People escaping on boats are a much bigger problem. An aircraft can be seen by radar at a great distance, boats cannot. Many will slip through. To combat this, a literal coast guard would need to be established: an armed militia of people patrolling the coastline both on land and in the sea in small boats. It would also be their job to find any washed up bodies and call in cleanup crews.
The containment cannot hope to succeed with such a huge border to defend. They must assume some will slip through. To deal with this, rapid response teams would be positioned some miles inland and ready to respond to anything which slips through, or any attempt to overwhelm the coast watchers.
Then you pray the disease doesn't jump species. Infected birds would be a serious problem. Killing the bird population preemptively may be considered, I do not know how that would be feasible.
Quarantined people would be stripped, washed and examined for wounds. Because of the extreme potency of any fluid contact, all their possessions and vehicles would be burned. Fortunately, because the disease is so rapid and obvious, people would not need to be quarantined for long. However, prudence suggests they be held for observation just in case.
Prudence would suggest keeping them all quarantined to a refugee camp, but keeping a few million people isolated would prove impossible. Instead, they would be allowed to resettle, but required to have regular check ups, discouraged from travelling, and required to register with the authorities. Given how people are reacting to the Ebola scare, refugees would likely be prejudiced by the locals. Probably anyone with a British accent would experience fear and shunning. Children would be kept out of schools. People would be refused by shops. Finding work would be difficult.
People involved in containing the outbreak would be required to wear boots, bite-proof coveralls, gloves, headgear and face protection when on duty. They would all carry radios. Civilians might use commercial equipment and cell phones. Military personnel would use their MOPP equipment and their chemical warfare training. All would be trained to always stick with their partner, to report in regularly, to report contacts immediately, and in the necessity of killing an infected person immediately.
Rescuing hold outs may be attempted. Short wave radio would provide a way to contact the mainland. Much of England can be reached by helicopter, and a safe landing spot such as the top of a building could be arranged.
An efficient, safe disposal system would need to be worked out. Questions would need to be answered. Does burning fully destroy the disease? Can it be spread in the smoke? How long does the body remain infectious after the zombie has died? Raids would have to be conducted to obtain specimens, fortunately dead ones, and taken to the quarantined facilities to be tested.
If it is found that smoke spreads the disease, this further complicates the matter beyond just disposing of bodies. Fires in the abandoned cities will inevitably break out and the smoke could reach France, the coast of France would have to be evacuated. To reduce the chance of fire, incursions would be conducted to secure utilities and safely shut off industrial facilities.
If it is found that the bodily fluids remain infectious long after death, the island may never be safe to return to.
Returning to the island to stay would be considered after perhaps 90 days when all the zombies should have starved, plus how long it takes for dead bodies to no longer be infectious, plus some padding for late infected hold outs. The problem will not be zombies, they will be dead. The problem will be 60 million bodies, their bodily fluids and other infectious materials.
This would be a purely military operation. The first priority would be to establish a defensible clean zone. A small area easily isolated would be chosen, such as the South West peninsula. Landing in Penzance, the A30 could be used as a defensible border with a clear field of fire. The city would be searched, block by block, house by house. Bodies would be handled by special teams and disposed of, probably by burning in the street.
Teams consisting of a sniper, a spotter and squad automatic weapon would be posted on rooftops to cover the advance. The sniper would take care of individuals while the SAW could cover packs. The spotter would remain alert, find targets, and make reports.
After Penzance is cleared, the advance would continue to cover the whole of the South West Peninsula. The the A30 between Longrock and Hale would provide the next defensible border. The advance would proceed, using highways as defence lines, across the island. Once the military has swept an area, trained civilians would follow them to perform a more through search.
During the long, tedious sweep and clear likely with little or no combat, there will be the temptation amongst the people on the ground to slack off. Maybe not wear all their hot, bulky equipment. Maybe not thoroughly check every building. This would have to be combated by keeping their shifts short and rotating people off the line frequently. This would require a much larger force than otherwise.
Clearing large cities would be a tedious nightmare. It would have to be conducted meticulously to get every room in every building, every sewer and utility space, every car and tunnel. The cities may never be declared safe. If that is so, it is questionable whether curious civilians can be kept out of the cities, and that would threaten the ability to return to the island.
If civilians are allowed to return, they would first settle in the easily defensible, easily searched, easily defended south west corner. Civilians would be educated about the disease, how to identify infectious materials, and what to do when they find it. They would be encouraged to wear long sleeve clothing and sturdy gloves when going into abandoned buildings or the wilderness.
Returning refugees to the island has a bonus effect: it quarantines them. Just in case any of them are carriers.
During the whole operation, the island would still be consider a quarantine zone, including the "clean" zone. An island transfer point would be established, perhaps on one of the channel islands. Ships and aircraft would be allowed to go from the mainland to the transfer island, or from the transfer island to Britain, but never from Britain to the mainland. Anyone returning would first have to pass through quarantine. Equipment would largely be on a one way trip. Aircraft and ships would be dedicated to the run between Britain and the transfer point.
There would need to be established a time when Britain is declared safe and the most odious/expensive elements of quarantine dropped. Several metrics could be used. One would be to simply count the bodies found. Another is to track the rate of bodies found. Another is to track the rate of infections. Once some percentage of the population is accounted for, the rate of bodies found falls below a certain threshold, and the rate of infection is basically nil, the island can be declared safe.
