Saw some pictures about the airport as well. It was around 1 cm (but definitely not more than 2 cm with temperature reported 0C or above through the day), which eventually melted by mid-day. Unfortunately 20+ aircraft (13 belonging to Wizz Air) had wait 4-5 hours for that to happen which does not mean that de-icing was at its best.
I can't wait for the day when it won't be +1C and half of our fleet will spend a whole day in Luton which will be a great Christmas gift for passengers of the subsequent 5-7 legs supposed to be operated by each aircraft.