Some of you guys seem to be missing FBW's point.
D&D coverage is from a limited number of antennas and specifically
'Covers most of the UK above 3000 feet MSL and in many (but not all) areas is good below this level'.
Whereas the unit you are tuned to (like London FIS or Manston, etc.) may well have a much better located antenna and be able to cover you much closer to the ground (although maybe not with the same VDF facilities D&D use).
If you can describe where you are, or are transponding, staying with your current service will avoid the potential that at your level and position you are out of range of D&D (in which case, you are relying on an airline to hear you, relay to their current working frequency, to relay again, for asking any questions, etc.). If you don't know where you are and don't have a transponder, then switching to D&D and hoping they can get a DF fix is probably a better bet.
I hear busy frequencies, but the squeal of two at once, shuts up most people. Key up, take a deep breath, say your piece, wait a moment and unkey. The frequency will then be quiet except for one Muppet who keeps on talking and when he stops, ATC will say 'all stations, standby while I work the Mayday' - and then he will go to you (periodically telling the new joiners to 'standby, Mayday in progress').