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The London Coliseum (also known as the Coliseum Theatre) is a theatre in St. Martin's Lane, built as one of London's largest and most luxurious "family" variety theatres. Opened in 1904 as the London Coliseum Theatre of Varieties, it was designed by the theatrical architect Frank Matcham for the impresario Oswald Stoll. Their ambition was to build the largest and finest music hall, described as the "people's palace of entertainment" of its age.
The theatre has 2,359 seats which makes it the largest theatre in London.
The Sadler's Wells Opera Company moved into the building in 1968. The Sadler's Wells company changed its name to the English National Opera in 1974 and today it is used primarily for opera as well as being the London home of the English National Ballet.
Grade II* listed. Link

English National Opera (ENO) is an opera company based in London, resident at the London Coliseum in St. Martin's Lane. It is one of the two principal opera companies in London, along with The Royal Opera, Covent Garden. ENO's productions are sung in English.
Website: Link
Wikipedia: Link

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