For future reference to others, this was v. doable.
Left central London with 100% 6:15am for 08:50Eurotunnel from Folkstone. Turns out there was a supercharger there whilst we waited and grabbed breakfast so even though unprompted, decided to add more juice there and then. 20-25mins worth.
From Calais(10:20am Local time arrival), 5 supercharger stops along the way, went straight to Lyon and got there for little later than 10pm. Was on 58% at this point.
Spend the night in Lyon, got up and went to Tignes with 1 more supercharge along the way, Got to Tignes with 40%.
Car stayed in a garage but not necessarily 'warm' or anything. Certainly not freezing cold either imo. 1 week later when it was time to return was down to 36%.
So didn't bother with public charging points there but I think you needed a specific 'membership card' to be mailed out in the post first to use those so would have been v.impractical /impossible for us.
Coming back into civilization from Tignes as you are going downhill the car did a fair bit of battery regen and we got to a supercharging point in 1 hour so was happy to recharge there with no range anxiety. And up north we went back up from another route and it was all good.