British Geological Survey (BGS) is the National Geoscience Data Centre for the National Environment Research Council and holds a wealth of geological data such as digital databases and maps, archives and materials collections. These datasets are available to search and view online through OpenGeoscience. Datasets include: groundwater chemistry data including Baseline Scotland; Borehole materials; Dicovery metadata; GeoIndex (a map-based index to datasets); Magnetograms; GeoScenic photo archive; Lexicon of Named Rock Units; Rock classification scheme; Web services (containing geological map data in open formats for mashups within open geo-browsers). There's also a link to the National Geoscience Data Centre (NGDC) which holds a comprehensive collection of geological and environmental information on the surface and subsurface of Great Britain, and offshore, which is available to the public, industry and academia.
Freely available data on internet.