X channel - wind speed and direction
First I must say that I am not well qualified to post on this thread - am still to go further over water than the Thames Estuary. That said I would consider ditching in water as a real last option and don't warm to the idea in the slightest. I would certainly put out a Mayday straight away if the engine failed.
As for planning the route and altitude I would pay really close attention to wind speed and direction so that I knew with as much accuracy as possible gliding distance in any one direction to a possible dry landing spot, trying to remember that the wind decreases (generally) as you get lower. That said if there was a very strong headwind that might warrant a lower altitude.