Also - according to the manual of the particular PA28RT I fly - the crossing at FL100 between France and the UK leaves a gap of around 20 nautical miles during which I would be outside gliding range to shore, my estimates of the fact that I fly at 140kts and best glide at 90kts gives me over 8 nautical miles extra before having bled off the speed to 90kts (I've tested this and in reality is closer to 12nautical miles) which would mean that I would need to have the engine stop during a 12 nautical mile gap (or during around a 5 minutes period) to not be able to make it to land (assuming no wind). I would have over 15minutes of gliding time to attempt restart, and call Mayday, also time to find a nice big boat to land alongside wheels up and fingers crossed be picked up very promptly by a crew member who according to IRPCS would be keeping a look out (and maybe someone from ATC may have passed this on to the naval emergency frequency so as to keep crews alert to the call). Also fingers crossed they have some nice whisky or rum or bourbon for the nerves!!
So this might perhaps sway you to consider the crossing? Or perhaps just follow the French coast up to L2K and cross there?