These green huts are known as Cabman's Shelters. They first started appearing around London in 1875 to provide cheap hot meals to cabbies. Even though the huts are pretty small, there's enough room inside for a little kitchen and some seats.
I wasn't particularly aware of these huts until I saw a documentary on TV. Apparently, there are now only 13 of them left. This one is located at Thurloe Place in Kensington, opposite the Victoria & Albert Museum. As you can see, it's located in the middle of the road, and cabbies, stopping for a bite, park their taxis behind it. I walked past the back of the hut and peeped through the window. The place was packed full of diners (around 10 of them) all squeezed in side by side, enjoying their meals.