There aren't too many 70 zones other than motorways in London so I can't say I've ever come across one either, but if by variable ones you're talking about ones like on the M25 then they work all the time.
They initially said they would only come alive when a lower speed limit was in force, then they changed it so they were always on but it was a high limit (around 90mph), but now it's just like a normal speed camera (allows 10% + 2mph - 79mph)
So beware if on a camera section of the M25, I just tend to set the cruise control on now whenever I go through there as I can't be bothered with the risk :)
Hmmm... Didn't know that...me thinks me needs to be a bit more careful next time I go to Heathrow ...ta 4 the info