I found the following on Concrete Block Associations web site (I got bored in work today)
"The materials to build the breeze blocks were very easy to come by. The cinders and clinker were obtained from the Rochester Gas Board who had mountains of the stuff. Great mounds of the many shades of mauve and brown which was the silica from the earth's crust and left as residue in the clinker after the burning process to produce the gas and coke. As far as the gas board was concerned this was an end waste product and he obtained the clinker
absolutely free."
The name seems to be 'agregate blocks' for whatever materials are used, and no hard and fast rules. I'm going to forget 'Breeze' from now on and call them concrete or thermalite !