CDG cave diver education has evolved over the past 50 years to equip experienced cavers to explore flooded cave passage. It is not formulated to train open water divers in sump diving techniques. The CDG consider it unreasonably dangerous to attempt sump diving without a good understanding of the nature and possible configuration of cave passages, which in the U.K. are small, complex and tortuous.
Unlike the U.S. there are (sadly) few large, long sumps in Britain, let alone with good visibility. This useful article by Schofield & Ryall describes the characteristics of typical British sumps.
A Dales Sump, (in good condition). Photo © John Cordingley 2000