The Eurostar from Waterloo or Ashford takes you to the Gare-du-Nord in Paris.
Since July 2017 a new 188-mile (302km) high-speed line is open between Tours and Bordeaux, knocking some 70 minutes off the journey from Paris to Bordeaux. Although the journey from London will involve a change of station in Paris – between the Gare du Nord and Montparnasse – this improvement will mean that the total travelling time from London to Bordeaux will be reduced to just under six hours (5 hours 55 mins) – nice and speedy, but still leaving time enough to savour the passing French countryside. From Bordeaux the fastest train to Belves, about 8km from Le Mas, takes just over 2½ hours.
Alternatively you can transfer to Austerlitz to catch the SNCF train from Paris to Limoges and on to Belves.
Rail Europe has a useful map of all the major European routes