National Council for the Conservation of Plants & Gardens
Through volunteer membership, the NCCPG seeks to conserve, document promote and make available Britain's great biodiversity of garden plants for the benefit of horticulture, education and science. Details of hundreds of plant collections, local groups etc.
Shows, gardens, plants, research, books, sales and the indispensable RHS PLANT FINDER
National Gardens Scheme
Now a national institution arranging the opening of some 3.500 gardens in England and Wales for charity. All listed in the famous "yellow book". Their site has a useful Garden Finder.