As soon as the siren sounded, Daddy would prepare to leave the house taking with him some cushions and the little lamp, plus a few odds and ends. Mother would then make a flask of tea or coffee and sometimes soup, some sandwiches and a biscuit tin full of cookies.
This family, at least, doesn't seem to have been in too much of a hurry to take shelter (even if the water was already boiling). Note also the use of the habitual 'would', implying predictability.
London: From what I've read about the London Blitz (7 September 1940 – 11 May 1941), there is little information on how much time people had to take shelter in London. Based on what my mother has told me - she was in London (near Lord's cricket ground) during part of the Blitz - the time between the siren going off and the first bombs falling was very inconsistent. However, my mother was only 8 or 9 at the time, it was a long time ago, and she wasn't in London for the whole duration of the Blitz so she doesn't know all the details.
Air raids (and thus sirens) could be on and off throughout the night and the warning systems were inefficient in London (though presumably they improved after a time), so it seems reasonable to conclude that predicting how much time one had to take shelter in London was difficult, at best. Also, there were apparently two warning sirens for "people doing important war work" so the 'rules' weren't the same for everyone.
In Birmingham, however, the only piece of evidence I've been able to dig up seems to imply that the time one had to take shelter was more predictable.
1. Can anyone confirm that, in London during the Blitz (7 September 1940 – 11 May 1941), the time one had to take shelter once the air raid siren went off was unpredictable ?
2. In Birmingham, was this time more predictable? If so, is it possible to say approximately how much time people had?
Note: I would also be interested in warning times in Liverpool, Plymouth or Exeter if anyone has any information on these cities. The time period, though, should be 1940-41 and I'm not asking about the 1945 V-2 rocket as it is common knowledge that this gave no warning.