With Warlord Games releasing the Operation Sealion book for Bolt Action, there has been a renewed interest in Britain's defences.
For a general overview, this Wikipedia article is a good start:
British hardened field defences in World War 2
Here are a few photographs of pillboxes I have seen.
This circular pattern pillbox overlooks Swanage harbour.
According to the above article, this is probably a Type 25. The corrugated shuttering leaves a distinctive shape to the outer surface.
This Type 24 pillbox is now being used as a shelter for bats.
This particular example's external brick shuttering with a concrete core.
This is the same pillbox photographed at a different time.