Singing humorous words to favourite hymn tunes is a longstanding habit throughout the British Isles, and is not considered in the least disrespectful. Apart from the Cwm Rhonnda parody already mentioned, everyone knows Lloyd George Knew My Father to Onward Christian Soldiers, just as an example. The unprintable Engineer's Wheel used to go to an old hymn tune, though now normally sug to A Frog Went A-Courting. Uncle George and Aunty Mabel (to Hark the Herald Angels Sing) is another pub classic. This is not a habit you will be able to stop, leeneia!