My experience of working London Info on a sunny weekend has been similar to Mariner9, and I did put 121.5 on the standby frequency as I couldn't get a word in edgeways on London Info whilst crossing the channel. On initial contact after leaving Southends frequency, I was asked to stand by, eventually called back after coasting out at Dover, told to call mid channel, but was coasting in to France before I could get a word in. Wouldn't have felt able to get a mayday in on that frequency on that occasion. They also seemed to be working traffic all over the place not just the channel. Heard someone around the Cheshire area on frequency going to the Isle of Man.
So whilst I agree its generally best to stay with who you are, London Info on a good flying weekend is an exception I think, unless they get more frequencies to spread the workload out.