© CRT Midi-Pyrénées / Dominique VIET

TGV trains from Paris serve Tarbes and Lourdes four times a day, taking about 6hrs. There are also nightly sleeper trains from Paris to Tarbes, and thrice-weekly sleeper trains that also carry cars (expensive, but useful time-savers on family holidays). A good source of timetable and practical information is Man in Seat 61 (Seat61.com). Eurostar can provide joint tickets for their own trains from London and onward journeys within France.
 
It’s about a 10hr drive from the ferry port and Eurotunnel terminal of Calais down to Tarbes in the Hautes-Pyrénées, with tolls costing about 60 euros in total. Or you can take a ferry from Portsmouth to Santander in northern Spain and then drive for about 4hrs. 

Tarbes–Lourdes–Pyrénées airport has daily links with Paris-Orly with Air France. However, you might find it cheaper (and easier) to take a budget flight to an airport such as Toulouse, Bordeaux or even Barcelona and hire a car there.