Not forgeting of course that those figures are for Still Air and the Mid point may not be mid channel, if you see what I mean!
I havn't yet taken my kids flying but one of the things I would want to do maybe next year is take them to France for the day. All this talk of ditching is scary when you think of not only getting yourself out, but the kids as well.
I may invest in a dinghy before I go, one advantage is that I fly a low wing, sliding canopy aircraft (Similar to an AA5). And it is only the Short route for me.
I seem to remember you can get up to FL55 over the channel, why stay at 4000', I would assume that at 5500' there would be virtually no "dead spot".
(I'm not going to do the calc, something that you are probably better off not knowing)