Ah triggered lightning.
All 4 companies use a Met Office forecasting tool which basically gives go areas, go areas if you can stay clear of cloud and in good visibility, and no go areas. If the computer model says your flight is to go through a no go area, then you just can't do it. The model is updated regularly, and seems to change it's mind very often which makes planning difficult.
Christmas parties. What are they? No chance any company would cancel flights on that basis.