Its nothing to do with aquaplaning - its to do with having no traction at the rear, compared to the front.
Liken it to driving round a car park with plastic trays under your rear wheels, it makes for a very unpredictable and uncontrollable driving experience. If you fit the new tyres to the rear, its easy to drive within your limitations, as you can feed in power and feel the loss of control. Fit new tyres on the front, and your driving feel won't change, giving you a false confidence going into bends or braking, where the rear end is extremely likely to step out.
Oversteer is controllable on a track or wide open, empty space - try and control it on a road with oncoming traffic and parked vehicles and you're likely to become unstuck.
Nevertheless, as I said, you're all driving £15k+ cars, why on earth would you only replace 2 tyres anyway!? It's all 4, or if you are **that** cheap, just the rears - there is no justification to change the fronts only.