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.
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.