The British National Corpus (BNC) is a 100 million word collection of samples of written and spoken language from a wide range of sources, designed to represent a wide cross-section of British English, both spoken and written, from the late twentieth century.
The full BNC (in XML) can be downloaded from the Oxford text Archive.
The BNC can be explored online via BNCWeb (hosted by Lancaster University). Registration required - sign up here.
The BNC can also be accessed on disc from the UA Libraries.