There are many ways of getting to Lourdes without spending too much money - here are a few ideas:
Ryanair operate from Stansted to Lourdes and Biarritz, and from Birmingham to Biarriz.
Easyjet operate from both Bristol and Gatwick to Toulouse.
FlyBe operate through the summer from Birmingham to Toulouse on Saturdays only.
You can get to Toulouse from most UK airports with KLM via Amsterdam, and of course Air France fly to Lourdes, Pau and Toulouse from Paris. There are also flights each Monday and Friday from Easter to the end of October with Tangney Tours from Stansted and with Mancunia from Manchester.
Trains to Lourdes leave Paris Austerlix and Montparnasse and take about 8hrs. Tickets for these don't go on sale more than three months before travel. This combined with a flight or Eurostar to Paris can be an easy way of getting to Lourdes. See SNCF (French railways) website for timetables, costs and booking tickets.
And if there are a few of you, why not consider driving? mappy.com is one of a number of on-line route planners which will show you the route from Calais / Cherbourg to Lourdes. Cross channel ferries can be arranged for under £100. The cost of petrol, tolls, ferry and a stop over half way could be less than even some budget flights.
If you want to programme co-ordinates into a sat-nav, St Michael's gate is: 43.098088, -0.051793