You can pre-order your ski passes with the lift pass office directly – you can order and pay on-line and have them sent to your UK address. Here’s the link
There are various choices for you when you consider which ski passes to purchase. These depend on your ability and how much of this awesome ski domain that you wish to explore. If you want to explore the whole of the Three Valleys you can buy a 3 Valleys pass for 6 days (or for however many days you wish). If however you are happy to stay closer to home you can buy just the local St. Martin/Les Menuires pass. Alternatively, if you want to stay close to home but enjoy a little more adventure take the Belleville Valley pass which gives you the local areas plus Val Thorens/Orelle. You can buy a Three Valleys or St. Martin/Les Menuires Family pass for 2 adults and 2 children (up to 18 years) but you cannot buy a Family pass for the Belleville Valley. Children up to 5 and seniors from 75 can ski for free. Pietons (people on foot) passes can be purchased for any lifts across the Three Valleys.