Having been watching this thread for some time I think it must be putting the fear of god into all who have not flown the short crossing before.
The Channel is the busiest shipping lane in the world, and most of us only fly across it in summer. In my aircraft I am out of gliding range of land for 90 seconds. The chances of your engine failing during this time are extremely remote. Calculate yours, plus your point of no return, based on the weather for that day.
I always play spot the boat. On a typical Saturday in summer you will probably be within gliding range of several yachts and you have a 30% chance of a warship. Any of these, plus numerous other options will have you on board in no time. This is by far your best survival chance.
Most aircraft float, at least for a while. There is a case of a DR400 which floated for over 2 hours (admittedly it had dry tanks). This aircraft was towed to shore and eventually flew again (I considered buying it 10 years later). Assuming you make a good job of getting the aircraft onto the water you may even have time to get out of a PA28, but I would not like to try it, personally.
Life Rafts
Based on a one week advanced open water survival course I did in the water around Jersey many years ago you had better hope the aircraft floats. None of the untrained fit young 20 something’s, which had not been previously trained, could get into the raft in moderately good conditions. That is a failure rate of 70 out of 70 (about 5 of us had done it before and we all made it, but we were all ideally dressed. This test was carried out in open water, in summer. Based on this the chances of your average PPL getting into a raft in open water must be very very low. I would guess my chances, wearing a lifejacket in place of a buoyancy aid, and with no wet suit, to be <50%.
In my youth I have flown for 1.5 hours across the North Sea and done the long crossing many times. I now only fly the short crossing, but I consider this to be very low risk, primarily because of the very short time you are out of gliding range and I do not fly it in winter.
I always hold a ditching brief before take off, as if your crew do not know what to do then you are in big trouble, particularly in a four or six seater.
Rod1