Yes, this is turning into a fascinating week down here. I'm struck by the dryness of the air in the subsidence inversion shown in the t'gram in the other forum and how this can mix down to clear the low cloud.
Whatever the exact processes, the fact that they can change cloud amount quickly and occur in local areas (giving sharp cloud boundaries) means that forecasters must study satellite imagery and current observations and be prepared to alter the forecast as they go. It was obvious it was going to start colder than forecast at Heathrow last night ( as my clumsily uploaded sunset photo shows https://groups.google.com/g/weatherandclimate/c/0ZV_E9zjgSs
). I wonder if model output was assuming low cloud or fog forming or the advection back of the frontal cloud? Certainly, fog formed in the more rural areas around here before midnight.
Verification of any model against Heathrow at night must also take account of the fact that it is a relatively warm site overnight. Cold air comes right up to built-up areas - last night's min for example at Teddington (Bushy Park), 1 mile from where I took the photo was -1.9C. At the normally colder Wisley to the south, it stayed above 0C. This may have been due to the approach yet again of the frontal cloud band from the south. At my location (between these two sites), a thin layer of Scu was visible overnight in the southern sky, tending to edge north until the whole view was obscured by fog in the second half of the night.