I would say the depth on that picture is about 4-5mm so quite legal.
Pity batch doesn't have that tyre on his car
Quite!
It'd be the safety I'd be concerned with, not the legality.
I've aquaplaned myself once and completely lost control for a couple of seconds, it's very easily done with worn tires. Fortunately I was going quite slowly so nothing serious happened.
Indeed your words rang true and I headed down to Bathwick. It looked to me that the circumference groove was close to the limit anyway. Turns out there are 2 indicators on Bridgestones, on at 3mm and one at 1.6mm.
My tyres are over 3mm so I'll use them up a bit more (mainly motorway) and change when they dip below 3mm (or before it snows, Bridgestone ER300 are like wearing slippers on an ice rink).
Thanks all.