The BGS recently publised the Geology of Britain website, which makes available a Geological map of Great Britain online in an interactive Map Viewer. They make available a 1:625 000 scale layer for Google Earth, and a the 1:50 000 scale version via WMS only.
Unfortunatly the WMS doesnt play nicely in Google Earth for some reason, so here is a hand tuned Google Earth layer to display the 1:50 000 scale Geological Maps:
Enjoy!