A quick summary of the alternatives

Flying : 
Quick, comfortable and potentially very cheap if you book early enough and generally the preferred option for travelling from the UK to Mottaret.  There are bus services from all the airports to Moutiers and then buses up to Mottaret. Car hire is good option, is often cheaper and gives you option to go to the big supermarkets/ski shops en-route.

You can fly to any of the four air ports below. 
Chambery is the closest airport but there are less flights here.
Geneva, Lyon and Grenoble are all about 2 1/2 hours drive. 

see

Train: 
Travelling overnight enables 8 full days skiing. 
There is a direct ski-train from London to Moutiers (at the bottom of the valley and then a short taxi or bus ride up the mountain to Mottaret).
Going via Paris can be cheaper particularly if you book direct and in advance, this also has the option of couchettes for the Paris to Moutiers leg. This is a very good option if you want 8 days skiing but prefer a good nights sleep.

Driving: 
A potentially cheap option if you have four or more in the car.  Can be the best option if booking late. 
It is about 8 hours without stops on the motorway from Calais to Meribel to Mottaret. 
A stay en route on Friday night close to Mottaret can enable an extra days skiing.