Only one airline flies from London direct to Lourdes airport, and that's Ryanair (from Stansted). However you can't solely use their service for a two-day stay, as it only operates on Tuesdays.
BmiBaby is the only other operator from the UK but their flights operate from Manchester, on just 2 days per week (Monday and Friday).
The easiest way to get from London to Lourdes is with Eurostar. On the random date I checked (5th July), the departure from St Pancras at 0922 will get you into Gare du Nord at 1247. Then you need to get across to Paris Montparnasse using one of these methods:
* From Gare du Nord, take Métro line 4 towards Porte d'Orléans, this will take about 40 minutes.
* Or you can take RER line B towards Robinson/ St Remy to Denfert-Rochereau, then Metro line 4 towards Porte de Clignancourt. This will take about 15 minutes.
* A taxi will take between 40 and 50 minutes depending on the time of day.
* Or you can take bus route 96 which will take about 50 minutes
From there, take the 1440 direct train to Lourdes, arriving at 2014.
The return service, two days later, leaves Lourdes at 1116, arriving at Paris Montparnasse at 1723. The departure from Gare du Nord leaves at 1913, arriving at St Pancras at 2034.
The fare is £89.50 in each direction, making £189.00 in total.
Chris