It is perhaps worth remembering that while Catholicism is not the dominant religion in the UK, those who follow this faith quite often spend a lot more of their time/money on matters of faith than the average UK resident and thus the numbers going to Lourdes is more than might be expected given the resident population number. Amongst those who are elderly or very ill and thus who are concerned with their mortality, praying at a shrine has a strong appeal. While not Catholic, I have seen the pilgrims at Lourdes - the religious devotion and atmosphere there is extraordinary
One can observe a similiar effect with the demand amongst Jews for flights to Israel or amongst Muslims wishing to go to Mecca or Sikhs travelling to Amritsar