In response to your enquiries about British COs.
There was a beautiful one on Clifton Downs (Bristol) near Brunel's Suspension bridge above the Avon Gorge. I've no reason to suppose it has disappeared since the 1970's when I visited it.
You walked up some stairs in a small tower to a room where the outside was projected onto a concave table made of some soft white mineral. Overhead was a handle so that the optical system could be rotated to change the angle of view and give the complete 360 degrees.
As to what the correct plural form is that rather depends on whether you are trying to speak English or Latin. I recommend you stick to camera obscuras.