I watched BBC news last night having got back to England as a pax for Christmas myself just 15 hours late. I think the explanation I saw was the Eric Moody Sky piece by the sound of it. It was simple, and accurate, explaining how extra spacing is needed to protect the ILS and that extra spacing is needed on the ground to avoid problems.
Interestingly when I tried to get out on tuesday (Londonderry-Stansted) we had thick fog (only one flight got in and out all day, apparently, but by wednesday morning we had a beautiful day in NI but the whole of the UK seemed to be covered. Seems I was lucky to get back so quickly, overall.