I think the guy who runs becketts bought the house next door and knocked them together and now having blocked up the front door to the leftward house in order to use the rightward door needs something to hide the block-up and creatively used an old sentry box.
Incidentally and a bit off topic, I had occasion to be in Bromley last night at the Warren which is a Met Police social club; on the way in on the right is a police box replica. I took a pic on my phone but it's rubbish and unpostable even after several hours on Picasa so you'll just have to take my word for it.