Actually, the last posted UK procedure is about taking at least 20 minutes extra fuel if you are not expecting any delays. More would be prudent. And of course that saying something along the lines that you are low on fuel is not recognized, which is actually true in the whole EU but may be different elsewhere. It is about using PAN or MAYDAY instead which is a much clearer declaration of an emergency than any ambigous sentence about lack of fuel. If one suspects that PAN is not enough, simply use MAYDAY.
Chicken and egg Denti, perhaps you know the back story better than I.
My understanding was that our UK Air Traffic Bretheren were getting a bottom lip with Callsigns declaring unjustified "Fuel Priorities and Panpans". They wanted to make it clear that in the civil sector the first call was meaningless and that in the case of declaring an emergency it was not done because the aircraft had gone below the min div fuel formulae but for the reasons BOAC has clarified. No delay=20mins was an attempt to give operating departments and flightdeck a real world guestimate to fight their corner in a world of fuel league tables so that they don't get close to declaring an emergency.
As a worthy Training Capt once stated, "I never carry extra fuel, only the fuel I think I might need". Wise words.