The first thing to note about Westminster Cathedral is that it is not Westminster Abbey!
The information in this post has come from Wikipedia:
Westminster Cathedral in London, England is the mother church of the Roman Catholic faithful of the Archdiocese of Westminster and the metropolitan church of the Westminster Province, located at 42 Francis Street SW1 in the City of Westminster. It is the largest Roman Catholic church in England and Wales.
The Byzantine church architecture by John Francis Bentley makes Westminster Cathedral a highly distinctive building.
The dominating external features are the great campanile, St. Edward’s Tower, 273ft high (top of cross, 284ft), and the West Front with its finely balanced pillars and arches.
The nave is the widest of any church in England and, because the Sanctuary is 4.5ft above the level of the nave, every part commands an uninterrupted view of the High Altar, with its imposing marble and mosaic baldacchino, on which light is cleverly concentrated. The richly gilt Crucifix hanging from the chancel arch is 30ft in length. On one side is the figure of Christ; on the reverse, towards the altar, the figure of the Sorrowful Mother. The Archiepiscopal Throne or cathedra, of marble and mosaic, is modelled on the Papal Throne at the Basilica of St. John Lateran in Rome.
The beautiful marble pillars are elaborately carved, with caps of white Carrara marble, no two alike. There are in all eleven side-chapels. Adjoining the Chapel of the Blessed Sacrament is a white marble monument of Cardinal Vaughan (d. 1903). The screen and gates in this chapel, surmounted by a gold pelican, are very beautiful. In a corresponding position on the other side of the Sanctuary is the Lady Chapel. The Chapel of St. Gregory and Augustine (the first on the right as one enters the nave) and the Chapel of the Holy Souls (the first on the left as one enters) are also complete; the former was the gift of Lord and Lady Brampton.
The cathedral continues to receive donations for the completion of the elaborate mosaics within. As of 2006, work is being undertaken to decorate the Chapel of St. Joseph.
Below the Choir is the Crypt, or St. Peter’s Chapel, also with fine columns. Here are monuments covering the remains of Cardinals Wiseman and Manning, transferred from their original place of interment at St. Mary’s Catholic Cemetery in Kensal Green.
The view from the Tower is much obstructed by nearby buildings and scenery. The tower is about 60ft higher than the western towers of Westminster Abbey, but is 30ft lower than the Clock Tower of the Houses of Parliament. Archbishop’s House adjoins the eastern end of the Cathedral, in Ambrosden Avenue.