Heres a bit about the type in your pic from the 'Pillboxes of britain and ireland' book, Fezzyben... (well worth getting a copy if you haven't got it!)
FC Construction pillbox, this structure is described on official air ministry plans, as the mushroom, the oakington, and the fairlop, and consists of an open circular concrete pit, with 3ft high wall, and domed concrete roof, cantilevered off a central pillar, a tubular rail holding 2 movable, clamped machine gun mountings is fixed below the parapet giving 360 degree field of fire.
widely used for airfield defence, over 50 examples on 25 diff. airfields known.
diameter;13ft
wall thknss; 12 inch.