Thunderball,
In conditions of high humidity but where condensation into water droplets has not taken place in free air, any small drop in pressure will cause local cooling below dewpoint and visible condensation will take place.
You can see the resulting water droplets in the wingtip vortices, flap edge vortices, and spanwise in the area of reduced pressure above the wings.
I wonder if this is what you saw?
ps: what was the date of your flight Ovda - Gatwick?
pps: Inadvertently replied before readind p2 of postings. All v interesting esp observation by D120A.