Ground beetles are generally found ... on the ground! Many are flightless, many are black in colour, and many are nocturnal and rarely seen by day. Many feed on small insects. This species, Harpalus affinus (it has no common name), is the exception to all of these rules!
Harpalus affinus is seen by day and is a wonderful metallic green when newly emerged from its larval stage but the metallic colouring wears off with age. It is vegetarian and feeds mainly on pollen from flower heads and so is quite often seen on flower heads rather than on the ground. It can fly which helps it get up to the flower heads and move between flower heads.
Whilst apparently not uncommon I cannot say I have seen it that often on my travels around Dorset.