So the question is, why nothing on one cold sunny day and everything on another? What's the science? I have real trouble imagining that the chisellers who run airlines would accept the cost of trucking any extra weight in the form of "stuff" to chuck out over London; that aspect in itself seems to bust any claims. So, what combination of factors (meteorological, fuel burn temps etc) would cause this one one day but not on another, comparable, day?
Basically it comes down to the temperature and relative humidity at the altitude the aircraft is flying at. The composition of the atmosphere at high altitudes does not on a day to day basis reflect what the composition of the atmosphere is at ground level. So while it may seem to be a similar day on the ground it does not mean the same conditions exist at altitude.
The contrails will always appear more readily ahead of a front just a bit like you see cirrus (high wispy cloud) ahead of a front.
My explanation is very basic but I hope it makes some sense. It would be easier to explain with diagrams showing how different air masses interact with each other.