How much 'extra' time do you allow yourselves to get to the airport, purely for traffic? We always work backwards from the flight time, add some time on for delays - and then find ourselves ready and out of the door about half an hour before that time! M25 can be a true pig - but only once have we not got to the airport rather too early - and that was the one occasion we were flying UC (no limo, obviously). How do you work out how much time to allow?
Seriously though, it depends where you are leaving from. I currently live up the road from LHR so can get there in 20 minutes, allowing an hour max for the Great orbital Car-park known as the M25

Thanks,
Sarah