what about commuting from say, South of France (Toulouse/Nice/Marseille) - that would be my ultimate goal of working with BA and living down there. Is that something that would be feasible within the first few years of employment as a cadet on SH?
Blindingly obvious advice but you need to look at junior rosters and commuting flight frequencies/timings between LHR and your planned abode before you can work out whether it's feasible or not.
If you're on a tight block of days off you don't want to have to sit around at LHR for hours on end, or perhaps have to stay overnight, waiting for the next flight home - that's a major PITA....How would your proposed commute fit in with early starts/late finishes? FWIW some people are doing well out of running B&B's on the Bath Road....
It certainly is "do able" eventually as there are commuters (Pilots and Cabin Crew) from all of the places you named, but most especially Nice, which is the best served of the above three cities in terms of number of flights a day, but they're mainly on Long Haul. There's a handful of short haul commuters from one other of the above three, but FWIW they're senior and/or part timers.