the best info on train times and connections in Europe is the Trenitalia site,which uses the HAFAS search engine.
The SNCF sites are not to easy to use, so are best avoided, and for train times in the UK, the First Great Western site is good.
the journey times I quoted are for a fairly relaxed trip. I have reached a point in my life where frenetic is just a word in the dictionary, but in fact, if you got your skates on and are prepared to risk tight connection times, it is possible to catch the 06.35 train from la Souterraine, and be in London around 11.30. You could have long lunch with friends or family, and still be back in la Souterraine by 22.45, but what a day!! As to costs, much depends on what date & times you choose. Train fares in France are constant, but Eurostar is notorious for hiking up prices on popular trains. Bookings for trains in France & Eurostar open 90 days in advance, so it pays to be organised well ahead. I have a SNCF Carte Senior, so I get 50% discount, but I think a full fare return to Paris from la Souterraine is around €64.00. On Eurostar, reckon around €75.00 return if you book early, no ticket change or refunds 2nd class, as much as £350 return Business class, late bookings. Take your pick!! I forgot to say that reservations are compulsory on all but local trains in France, and on all International trains
I have been a closet railway enthusiast all my life, but I decided to come out when we moved to France! Limousin is a fantastic place to live, but you can catch a train to anywhere in Europe from a lot of local stations, for example, there is a sleeper/couchette service to Barcelona everyday at 23.56, arrives in Barcelona Sants station at 08.34. Or you could go to Geneva, via Lyon, and then take the "Cisalpina" to Milan, which goes along the shores of Lake Geneva to Lausanne, across the Alps and down to Domodossola, around the shore of Lake Maggiore, ending in Milano Centrale.If you enjoy train travel, this is a trip to take before you die!