The information is all rather old hat I'm afraid.

http://www.youtube.com/watch?v=mEXD-_O_MpY

(although I can't see the UK authorities today being so organised considering the recent poor weather)

If you do decide to move quickly (assuming the heat and blast wave hasn't turned you to toast) its best to traverse 90 degrees perpendicular to the prevailing wind direction assuming of course you have a secure bug out location to head towards and can stay there in the fall out shelter for 14 days without resupply.