I’m SE based and 10-12ish hour drive vs 1 hour to get to airport & parked plus shuttle to terminal + 2 hours before flight + 2 hour flight + 1 hour at least collecting luggage + 2 hours with a transfer faffing about stopping everywhere or 1 hour drive but at least half an hour getting to the car rental and sorting out the car.
So, worst case with flying, and that’s assuming no flight delays, it’s not that much quicker than driving door to door.
That said, skiing I tend to fly but luggage costs are racking up these days taking skis.
Plus Les Deux Alpes is close to Grenoble airport so less of a transfer in theory.
Bike trips I’d prefer driving as so much more flexible with what I can take and don’t have to pull the bike apart.
Anyway, flight prices have gone up a lot. The pound being bad one reason for a start. No idea where it will go what with Brexit and stuff.
Train, looks like Eurostar to Paris and SNCF to Grenoble. Then transfer. Quick look and about £150 return just based on one person (interested myself at train prices. Last time I did one was a very long time ago Eurostar overnight ski train).