After the apocalypse ??

[Tartar Horde]

That's a good point about outrunning the radiation. Any distance from the radiation would help, and given the wind directions in the UK anyone on a boat would be wise to head SW into the Atlantic when the plants go boom. The majority of Nuclear power station in the UK are Magnox reactors, the little piece below explains a little about their "safety" features.

The Magnox reactors were considered at the time to have a considerable degree of inherent safety because of their simple design, low power density, and gas coolant. Because of this they were not provided with secondary containment features. A safety design principle at the time was that of the "maximum credible accident", and the assumption was made that if the plant were designed to withstand that, then all other lesser but similar events would be encompassed. Loss of coolant accidents (at least those considered in the design) would not cause large-scale fuel failure as the Magnox cladding would retain the bulk of the radioactive material, assuming the reactor was rapidly shutdown (a SCRAM), because the decay heat could be removed by natural circulation of air. As the coolant is already a gas, explosive pressure buildup from boiling is not a risk, as happened in the catastrophic steam explosion at the Chernobyl accident. Failure of the reactor shutdown system to rapidly shut down the reactor, or failure of natural circulation, was not considered in the design. In 1967 Chapelcross experienced a fuel melt due to restricted gas flow in an individual channel and, although this was dealt with by the station crew without major incident, this event had not been designed or planned for, and the radioactivity released was greater than anticipated during the station design.
Now there are a lot of assumptions here given by the Nuclear industry, but not one of them has figured in a total breakdown and loss of power to all primary and secondary systems. It doesn't matter if the core is water cooled or gas cooled, there has to be circulation of coolant to stop rods heating up. In a total collapse and power failure the only thing controlling the time it takes for the Nuclear power station to melt down is the amount of back up fuel supply TPTB have to keep the secondary systems running. What happens when the power goes off and stays off? the "cores" may well be "shut down" but this is a misleading phrase as it implies there is no activity within the core, even if the core is shut down it remains extremely radioactive, hot and deadly to all life.