I've done this 3 times now. each time overnight from St Pancras Friday arriving early Saturday and departing Moutiers Saturday arriving London Sunday morning. Each time skiing in Meribel
You get 2 additional full days of skiing, we arrived in Meribel by transfer bus from the station before 8AM and left after 6pm on the following Saturday
If you are self-catering you can take as much food as you can carry as there is effectively no limit to luggage
Food is provided for premium seats (we took M&S sandwiches etc when not in premium)
The seats are reasonably comfortable, bigger than economy flight seats but a good nights sleep is far from guaranteed
There is no alcohol allowed on the overnight service, we saw a chap with a 30 year old malt have it confiscated even though it was for consumption in their chalet.
You are hanging around a lot on the return leg, the train leaves around 10pm from memory, it's cold and pretty much everything is shut in Moutiers station.
Check-in is not the quickest.
When the train stops in Moutiers (and for you at Ashford) it is not the end of the line so you have to be smartish getting all you kit on and off the train, there's a lot of people and a lot of kit, you just need to be organised and have all your bags to hand.
It may sound a little negative but I'd happily do it again and not just because the airports serving the alps are simply awful and Gatwick is not much better. Overall journey time is a little longer but so relaxed and with a few mates you can have a good laugh. It is possible to sleep well (take a pillow/next support thing) and we did a full days skiing on arrival (after finding somewhere for a full English) and departure.