WHBM.
Monarch only supply the aircraft, they do not sell Lap Land Flights to Joe public.
So it makes no difference that as you quote they are LCC, it was a charter flight as is every Santa flight at this time of year.
You might also find that they were already Sub Chartering aircraft on Friday and Saturday.
However it was a bloody big own goal by whoever made the decision, someone should have known it would look bad in the press, after First Choices problem a few weeks before.
