Well the BrickPillbox here was part of the Chain Home Low Radar site at Happisburgh which are all Brick, built so logic would dictate that this would also be the case!!1
Aye, Chain Home sites all seem to use brick for main defences, I'm discovering.
Aberdeenshire beach pillboxes appear to have used wood for shuttering in the main. I assume because it was easily portable, and construction took place in some pretty inaccessible locations.