Scythe, the likelihood of getting enough of a warning before a nuclear strike to enable an efficient evacuation of the population likely to be hit is very small.
I fully get what you are saying, but how do you move and re-house up to 90% of the population?
The UK's transport system are already at gridlock at peak times.
Yes anything "could" happen, but that does not mean that TPTB will adopt a 1940's strategy in the 21st Century.
Mary, I agree. But I think we'll have warning of something, because we'll have some kind of warning about political tensions building. It is more likely to just be a movement of children, than just adults. Moving them out of the cities would be simple enough. Even with just a few hours of warning, they'll be easily put on a transport system and moved as far as possible from the city centre.
Dev, we may not adopt a 1940's strategy in the 21st Century, your right. But we have already (in the last 5 years) seen petrol rationing in some areas, curfews around the riots a few years back, and things like that. Even though we may not necessarily adopt it, since we've already opted to go down a few of those routes from the 40's and earlier, in the last 10 years, then there is still the chance we may well adopt the policy.
As for the gridlock...really? I've not seen much gridlock down here. Maybe at rush hour in London, or parts of very large cities, but not all the time. To take over a train and send the people this way...pretty likely route to go down. Not necessarily motorways, because of the crash potential. More likely to be train routes.