Cap413 says to always make the first call to the station you are already talking to.
But in the circumstances, I would go to D&D. Although there may be 4 others on London Info, there may be more maintaining a silent watch on 121.5.
If you were sighted by another aircraft, that would be very beneficial. If you were at FL65, you've got over 10 minutes and your track should give a good chance of being rescued as quickly as possible.
Personally, I would rather talk to Manston or Solent, than London Info, as both of these have a LARS.
The one thing to remember is that circling aircraft can only guide SAR in. Your best chance if a ditch is unavoidable, is to do it just in front, to one side, of a yacht, shouting as you pass it.