I'm no expert but my money would be on a pillbox of some kind given its position. Judging by the pictures and description it could be for a larger weapon (a/t gun)?
I believe pillboxes of this type were quite rare, usually being designed for a 2pdr a/t gun although invariably fitted with anything available, usually a leftover from WW1. There is a fine example at Brandon Creek on the Norfolk/ Cambs border on the old GHQ line. It could even be a locally built bespoke emplacement for a locally sourced weapon. Shades of Dads Army but remember these were desperate times and anything was better than nothing.
If you visit the tank museum at Bovington you can see a reconstruction of a 'tank' which is basically a 3 ton truck with a sandbag pillbox built on the back and which does not conform to any of the standards required of a successful AFV, namely firepower, protection and speed!
My uncle (ex Irish Guards) spent most of the Battle of Britain defending Northolt in an Austin Beaverette- an open top austin civilian car with a few sheets of metal bolted on and a 0.5inch Boys anti tank rifle-hardly a match for a Panzer!