I'm going to take a guess and say we don't have these. What are they?

It's a cupboard (closet), built into the house and often in or near the bathroom, where the plenum for the hot water system lives. These hot water tanks, about the size of a dustbin (garbage bin), are usually very well-insulated, but not infinitely so, so the cupboard remains warm all day, as compared to the rest of the house. It's a good place for that final drying of clothes which come out of the drier not quite dry, for proving bread dough, and for fixing damp consumer electronics.

Do I need to take one step back and explain how British heating systems work in the first place? What usually happens is that the heating comes on in the morning, heats a tankful of water, and then goes off again. Unless everyone has lots of baths, that reservoir of water is enough to last all day, and then in the evening the heating comes on again and refills(*) the tank with hot. Occasionally you see heat-on-demand water systems, especially in smaller houses or flats built in the last ten years or so, but they're not as popular as hot-water-tank systems -- especially with families, as tank systems have the advantage that the water coming from one hot tap (faucet) doesn't change in temperature if someone else in the house turns a hot tap on.

Peter

(*) Which is a slight oversimplification: the hot water tank is always full of water, as it's connected to the water main at the bottom. As hot water is drawn out from the top, cold water enters at the bottom; sufficiently little thermal mixing occurs over the course of a day that the temperature of the hot water doesn't really decrease until it's all used up and you start getting the cold water. If this happens and you don't want to start the main heating, most hot-water tanks have an "immersion heater" which is a great big electric heating element that heats the top portion of the water; this is reserved for "emergencies" as the main heating -- whether gas, oil, or coal -- is usually much cheaper per hot-water joule than electricity.