What where the road conditions at the time? It sounds to me as though what you may have experienced was the ABS operating. If the pads are worn down to the metal you'll get a grinding noise every time you brake, not just under hard braking.
Road conditions were normal, dry. I certainly didn't skid.
I had a feeling ABS might have come into it, as it was almost as though the car just stopped braking in the final second or so, or at least with much less pressure - as the grinding noise was happening. It felt like the brakes had been released (hence the lurching forward feeling), yet the car did eventually stop.
I too would have thought that I'd have the grinding noise all the time when braking if the pads were completely worn out, but that's not the case. Braking at slower speeds is perfectly normal (other than a bit more give in the brake pedal over the last few days or so).
I have just done a visual inspection now, and its hard to tell, but there doesn't look to be much brake pad material on any of the brakes, and kinda looks like the metal 'frame' of the brake pad would touch the disc during braking. Not 100% sure here, as it was hard to see.
But, on one of the front discs, I can see that the inner half of it is shiny, and the outer half of it is rusty, and I can't see any brake pad material on this one!!