Devon and Cornwall will be inundated with refugees? Ifs, maybes, and conjecture

[Devonian]

But what type of event do you envisage would cause circa 20M people from the South East, to still be alive, and to just up and leave everything behind, AND for the rest of the UK to be uneffected?

You (or someone else) previously mentioned a dirty bomb, but that would only affect a VERY small area and would not cause mass evacuations?

A full on nuclear attack would wipe out the popultion anyway, as would a Tsunami. Anything that is not immediately life threatening would be ignored by many.

Whilst the numbers being quoted may be correct, the scenario's are are not. I cannot see any 'realistic' reason for the whole (or even a large part) of London and the South East to be evacuated.