Type II Holdover
BOAC - obviously I have too much time on my hands, because I went and looked it up. According to my Ops Manual, type I fluid can give as little as 6 minutes of protection, so there would need to be a de-ice bay near the departure runway. Type II fluid however, in a typical 50:50 fluid/water mix, is good for 20 to 45 minutes of protection against freezing fog. Ample to get from the parking stand to the runway, I would have thought.
But in any case, I don't dispute that it is difficult, but the original question (particularly interesting given that my Ops Manual apparently tells me HOW to depart in freezing fog) was - is there anything that prohibits trying it?