I flew a couple of times to Lourdes in the late-'80s/early-'90s with British Airways--cannot remember what aircraft it was, but it wasn't small and it was packed.
Lourdes was bidding for Ryanair service but lost out to nearby Pau.
Biarritz too. Irish Nuns are regulars on the Stansted-Biarritz service too, although Pau does make more sense due to its proximity to Lourdes.
The only explanation I can think of regarding the decline in direct services from the UK to Lourdes might entail stating the obvious; religious following is on the wane in Britain (I don't mean to sound reactionary). That said, I know quite a number of people in London who have gone to Lourdes to help out in recent years, of whom several made it an annual commitment. Some of them fly to Biarritz with Ryanair, others Pau.
How do you say "12 months" in Estonian?