These minefields had three main roles: protecting important coastal convoy routes, preventing U-boats from entering the North Atlantic, and protecting the entry into major ports. Generally speaking, these were divided geographically - those on the east coast protected the important coastal routes between Edinburgh, Newcastle and London. These fields were also an anti-invasion measure, as the British thought that East Anglia presented a tempting target for invasion.Those in between Scotland and the Faroes blocked the quickest route for submarines wishing to enter the North Atlantic from Germany. These minefields would also continue to the north-west, with mines laid in the Denmark Strait between the Faroes and Iceland. Finally, those on the west coast, around Bristol, Liverpool and Glasgow, protected the main Atlantic convoy routes, which ran into these cities. Mines couldn't be laid along the whole length of their routes, but instead protected the areas where convoys would form up.

Target-wise, the western and northern fields were mainly aimed at submarines. German surface raiders were thought unlikely to try to penetrate these regions, given their proximity to major naval bases. Submarines could, however, lurk there to ambush convoys, or try and sneak past RN patrols. The eastern fields, however, aimed to destroy submarines and light German vessels, such as the E-boat and S-boat. These were commonly used in raiding operations against small coastal convoys, especially in the Channel and along the East Coast.

The defensive minefields were not very successful. The East Coast barrier only sank a single torpedo boat, and mining operations were cancelled in 1943. Minefields in the Western Approaches sank 2 U-Boats, while the Northern Barrier could only claim one probable. However, several submarines may have been sunk there, as the position of their sinking is unknown. The Northern Barrage was referred to as "the least profitable voluntary major undertaking of the war" by Admiral Tovey, the commander of Home Fleet.

Sources:

Churchill's Navy, Brian Lavery, Conway, 2006

British Naval Minelaying in World War 2,Lieutenant Commander Geoffrey B. Mason, naval-history.net, 2006