"The mobile network doesn't fail during a major incident"
The whole concept of this exercise being based on the loss of voice services whilst data services continue as normal is utterly farcical (no surprise there then). OK the core government data networks and whatnot hopefully don't run over the vanilla DSL network and some of them hopefully even run over non-BT networks, but that aside...
"The mobile network doesn't fail during a major incident"
Do you know what a lot of the smaller cells use for their connectivity back to the cellco core network these days?
I'll give you a hint: it's not microwave like it used to be in the early days.
Time's up, and the answer is: DSL, over telephone lines.
Now, OK, typically we're only talking smaller fill-in cells, which are typically only added to provide extra capacity or extra coverage... there will hopefully be microwave-linked cells surviving even when the DSL-linked cells go dead.
Many of those DSL lines will be over BT DSL services, ie BT 21CN DSL needs to work or the cells are dead. Yes that again. Got the message yet?
You might also want to read the September 2006 London Regional Resilience Forum report [1] into the lessons from the 7/7 bombings in London in 2005. Re the (non?)usefulness of the ACCOLC selective access mechanism it says e.g.
"The ACCOLC system was invoked for a short time in a one kilometre radius of Aldgate. It subsequently became evident that the ACCOLC system was not currently accessible by all Cat 1 and 2 responders that may have a critical need for it. In any event, the use of ACCOLC procedures could themselves be counterproductive because the public relies heavily on mobile telephones as their primary means of communication and would want to use them in a crisis to reassure family and friends."
Hopefully more Cat 1 and Cat 2 responders now have access. Would you want to count on it?
[0] http://news.bbc.co.uk/1/hi/technology/5140358.stm
[1] http://www.londonprepared.gov.uk/downloads/lookingbackmovingforward.pdf