Hypothetical Look at London after 3 days without power

[Skvez]

(1 November 2011, 11:14)NorthernRaider Wrote: The hospitals and frozen food stores have back up power supplies as do SOME telephone exchanges, but the amount of fuel stored varies between 4 and 7 days worth

Most backup generators have a fuel tank with between 4 and 12 hours of fuel.
Hospitals and sewage works may have some additional storage but I'd be very surprised if any shops have additional fuel on hand.
Telephone companies are legally required to have sufficient back-up to keep the phones powered for 24 hours after loss of mains power (although these days lots of people have handsets (typically wireless)that require separate power rather than line power).
Most electrical outages in the UK are less than 4 hours and so that's what the plans take into consideration. Their idea of worst case is (after a few hours) close the shop, get the shoppers out in an orderly manner and assume it will all be fixed for tomorrow.

However short of an EMP I don't see a scanrio that would lead to an instant total 3 day+ loss of power in London.
Most SHTF scenarios would experience increasingly worse intermittent power.