The ones up by the speaker are also noticeably lower resolution than the other cameras. There are currently 10 cameras in the commons, 2 at the back, 6 along the sides, 3 each side, and the 2 q-balls each side of the speakers chair.
From the pictures I've seen it looks like the main cameras were upgraded not too long ago, but the existing ptz's were reused. The cameras and associated equipment are these days owned by Parliament, and controlled, to their requirements, from Millbank. Because of the amount of use they get, I suspect they'd tender for reliability over compactness. Especially since the precedent of the larger cameras was set many years ago, there's no need to try to hide the current cameras.
So who controls the cameras as you mentioned Milbank?
Don’t let anyone treat you like you’re a VO/SOT when you’re a PKG.