On the subject of people hearing Mayday calls from a VFR pilot when the radio is busy, would suggest making contact with a radar service before coasting out and, if something goes wrong and the circuit is busy, squawk 7700 and they will call you.
There is no universal radar coverage at typical VFR OCAS altitudes across the whole of the Channel - or the North Sea for that matter. Only if you use the Dover-Calais route can you be under radar surveillance throughout the crossing.
http://www.ead.eurocontrol.int/eadba...2013-04-04.pdf
Note that not all units are 24/7. Manston is supposedly 7 days a week, but 9-17 only. Plymouth is only Monday to Thursday 9-17 and Friday 9-14. (UK AIP ENR 1.6)
If I cross Calais-Dover or Koksijde-Dover I prefer to talk to Manston, if possible. But for all other routes you don't get a lot of choice. It's typically London Info or nothing at all.