Gurtrude
You miss my point the transponder should go to 7700 as soon as the problem starts and this will chances to alert ATC but it will not give a radar fix on the actual ditching position, the fix that ATC will have as the aircraft goes below radar cover is likely to be greater than a mile from the actual ditching position.
The advantage of a GPS based R/D position report is that the last report can be broadcast just as the aircraft is about to hit the water, this way anyone monitoring 125.5 can relay the most accurate fix to ATC. with any luck on a normal day across the channel any number of pilots could punch the numbers into the GPS and be overhead very quickly, this gives the best chance of spotting a person in the water and helping the SAR assets who can use the same fix to get directly to the crash site.
If a person fails to get into a liferaft and has not got a PLB them being spotted by the Mk1 eyeball are very slim and death by exposure is likely within2 hours in the summer and and about 15-20 min in the winter so speed is essential, the whole idea is to try to take the search out of search and rescue.