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Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link

Her Majesty's Naval Base, Devonport (HMNB Devonport), is one of three operating bases in the United Kingdom for the Royal Navy (the others being HMNB Clyde and HMNB Portsmouth). HMNB Devonport is located in Devonport, in the west of the city of Plymouth. Having begun as Royal Navy Dockyard in the late-17th century, it is now the largest naval base in Western Europe and is the sole nuclear repair and refuelling facility for the Royal Navy. Shipbuilding ceased at Devonport in the early 1970s, but ship maintenance work has continued: the now privatised maintenance facilities are operated by Babcock Marine who took over in 2007.
HM Naval Base Devonport is the home port of the Devonport Flotilla which includes the largest ship in the Royal Navy HMS Ocean and the Trafalgar-class submarines. Devonport will no longer be used as a base for attack submarines after these move to Faslane by 2017, and the Type 45 destroyers are based at Portsmouth. However, Devonport retains a long-term role as the dedicated home of the amphibious fleet, survey vessels and half the frigate fleet.