I think this confirms that the pilot in a Lancaster definitely had a seatbelt. From an unpublished manuscript written after the war by S/Ldr DPS Smith, of 467 Sqn - my great uncle's pilot and the only survivor from the crew. This is his description of the moment they were shot down over Lille on 10 May 1944:
We were just about to drop our bombs when everything went hot and dry and red. When the flame had gone out, I was still in my seat but could feel no aeroplane about me. I immediately released my seat belts and then my parachute. It seemed to open immediately...
I've asked another veteran I know (a rear gunner) if he can add to the discussion.
Adam