Short answer: Keep whatever ATC squawk you have been assigned unless told to change it. Open FIR: Squawk 7700: it makes all the bells ring at D&D and you will have the attention of any radar controller in the area.
Ref MAYDAY and PAN: Put another way, you will ONLY get preferential treatment on the approach if you declare an emergency. PAN or MAYDAY (whichever you think best) is therefore the way to go. For the benefit of some, there is no such thing as a 'Fuel Emergency' in UK airspace!