Th best way is to fly into Nice, and rent a car from Nice Airport to Saint Andre les Alpes. There are ample international hire car companies available. EasyJet flies into Nice from London, as well as most of the major carriers (BA, AF, LH etc.)

Alternatively, via public transport, take the TGV to Aix en Provence, then bus to Digne les Bains, from Digne, take the narrow gauge "Train des Pignes" to Saint Andre les Alpes.
From Nice, take directly the narrow gauge "Train des Pignes" right up to Saint Andre les Alpes