Is there a reason why none of the shops sell 18mm deep sockets? Halfords, Screwfix, B&Q and the like seem to sell every size except 18mm in a deep socket.

Not much comes in 18mm AF…
What are you trying to undo?

18mm isn’t a very commonly used size. Machine Mart sell them if you have one local. Otherwise it’s internet purchase from one of many suppliers.

Think I’ve found one in a Halfords automotive set – trying to under a crash bar on a car.