The only problem with the premise is that in the real world if all the nuclear power plants were left unmanned for a week or more, there would be nuclear melt downs all across the planet, like Chernoble in the former USSR. This would cause radiation across most of what is left of the population killing all the remaining survivors. France next door to Britain , has about 45 nuclear power plants. Imagine all the radiation that would be produced by that many power plants left unattended, and they melt down.