You can find some official British thinking on this at Uk Resilience. It sounds like the police cannot usually require you to leave home, and they can't require you to go to a government refuge.

The London Major Incident manual also makes interesting reading. Some of it seems very buearocratic, but with hard experience of monumental cock-ups lurking between the lines.

Eg: "Within the capital, each of the emergency services and separate police forces have entirely independent radio communications systems. That is not to say that they are totally incompatible but certain procedures need to be adopted to achieve mutual communication." And "It cannot be over-emphasised that the best operational communications will always be conducted verbally between service representatives on site." Reassuring.