A Seneca can surely do Luton to Nice nonstop? About 600nm. A TB20 could do that and back to Luton without landing, down to nearly zero fuel.
Its about 660 nm Luton to Nice the Seneca will fly 5 hours to dry tanks at 160 kts or 4 hrs with an hours reserve.
Four hours will take you 640 nm.
Take them up into oxygen land and yes the TAS will increase to around 200 kts
We had a headwind which gave us a GS of around 130-140kts go figure and flew at around 10K?
Pace