As owner/pilot of an N-reg PA46 with FAA IR flying within Europe on a regular basis, I might bring some insights into the discussion
.
To me the two most important questions are:
1) why was he flying at 5000 ft?
The distance between Nantes and Cardiff is around 270nm. Even though the winds were quite strong (>60kts headwind),
I would always choose an altitude between FL160 and FL240. This would allow for a 2h flight and give enough glide distance
in case of an issue. Also at that altitude I would be out of ice completely.
I fear he was aware not to comply with all rules and tried to play it low key:
Flying an N-reg aircraft IFR in Europe across two countries is only legally possible if you hold an FAA IR Licence and maybe
if he holds both French and UK EASA IR. Very unlikely.
Also the flight sounds to me like a commercial job more than a shared cost. You take a football star across the channel at night
during winter for fun? Very unlikely. To my knowledge, you can't operate a single piston IR commercially. Neither EASA nor FAA.
Possible supportive facts could be that
a) the transponder was off or at least no 7700
b) he did not declare an emergency on the radio
c) he did not turn on ELT (you can manually turn it on before impact)
2) what would make a pilot ask to go down from 5000 ft to 2300 ft over water?
There is only one reason coming to my mind: Ice. In case of engine or electrical failure, you are not asking for lower level, but vectors to nearest field (ie. Guernsey).
The PA 46 is equipped with a variety of deice systems, but you need to be very proficient to use theme properly. Especially following systems are critical, assuming that pitot/stall heat are working
a) Alternate Air: If you forget to open alternate air, the filter clogs and the manifold pressure drops => low power
b) Prop deice: If you forget to turn it on or it is not working properly, the aircraft will start to shake like hell => low power, less control
c) Wing deice: If you forget to turn it on, or at the right time or not working properly, the aircraft will become heavy quickly and the controls bad => less control
In any of theses cases, if the autopilot is still on, it might stall and/or disconnect at the worst possible moment => loss of control
But even the autopilot is off, you will need good stick/rudder, IFR proficiency and luck to come out of this...