What make were the front tyres? Look like Blockleys. As soon as the load goes off the rear, that tyre is mostly just keeping the keel of the ground, so I would look at the fronts for blame.
Fronts are week old Blockleys and the rear is a much older Michelin Cross Climate which was just under 3mm in the centre and a bit over at the shoulders. I had already done about 150 miles on Saturday, all in the rain and including lots of standing water - think deep puddles - without any issues apart from visibility till I gave in with the goggles and stuck my normally track only full face helmet on. From my experience on the day and what the policeman said about that particular stretch of road there is something about the surface or geometry of the motorway there that produces more than its share of that kind of sudden aquaplaning. The "lump" of rain was extremely heavy and from the way it snapped I will hazard a guess that one front and the rear aquaplaned at pretty much exactly the same time while the other front stayed on the road surface but with lots of drag from the water depth so the whole car just pivoted around it. I hit the opposite lock stop pretty damn quickly but there was no purchase on the road at all and I might as well have not bothered.
I'll also add that until the motorway when I was on the particularly wet bits I was frequently making gaps for myself to check for grip and keep the brakes dry so I had a fairly good picture of the grip available.
