I’m not sure if anyone has considered the radar station at Crossma. They track all marine traffic in the Channel and Channel Islands water with very sensitive equipment. I would imagine a small aircraft, at low level, would have painted on their screens right up until it was shadowed by Alderney or the Casquets reef thus giving a pretty good position of the actual ditching. I stand to be corrected but there is more than ATC radar cover in this area especially with the sensitivity of a nuclear power plant on the coast.
Standing by!