From the World Heritage inscription:
Westminster Palace, rebuilt from the year 1840 on the site of important medieval remains, is a fine example of neo-Gothic architecture. The site – which also comprises the small medieval Church of Saint Margaret, built in Perpendicular Gothic style, and Westminster Abbey, where all the sovereigns since the 11th century have been crowned – is of great historic and symbolic significance.
Westminster is London. When you think of London you think of Big Ben, Parliament and Westminster Abby. In addition to the abby you can also enter parliament, which is something I did back in 1999 when I visited London. If I had to list one thing everyone should do if they only have a brief layover in London, it is to visit Westminster.
View my complete list of UNESCO World Heritage Sites