It depends on the whips (!)
Most Commons business is managed by the parties (Labour v Conservative/LibDem coalition) "pairing" their MPs so they can be elsewhere doing business or having lunch. On routine stuff little more than those down to speak are in the chamber.
This caused no problem before TV was introduced, since no-one could see the empty chamber! Now they tend to huddle around the person speaking (donutting!) so that the chamber appears full to the camera.
On critical debates ( a "three-line whip" since the item is underscored three times on the order paper) MPs are there on pain of death!
It is quite complex but that is the guist of it.
