They are very cheap and most cook shops stock them. Some of the fish eaten in Europe and North America are the same, but many are different. When the fish is the same we call it by the same name. What North Americans call shrimp, the British call prawns. British shrimp are really tiny, and are usually served as 'potted shrimp', in solidified clarified butter.
British crayfish are also much smaller than the North American variety.
The difficulty with the terminology for meat is not transatlantic so much as home grown. In the U.K. there are regional variations in both the way a carcass is cut and the names for the different cuts. Additionally, French cuts are often used. In one respect British is plainer than American. What North Americans call 'Variety Meats' we call 'Offal'.