Dyer's Greenweed (Genista tinctoria)
It grows in grassy areas on heavy soils and flowers from June in to August. It grows in just one of the meadows at Durlston and also in a couple of cuttings on the Swanage Railway (thanks to Ted Pratt's book). It may occur elsewhere in Dorset but I have not seen it anywhere other than Durlston.
As the plant's name implies, it was used for dying material, even the scientific name suggests that too.
Notice too the lovely Quaking Grass in this picture.