This is a bush-cricket (family Tettigoniidae). Where did you find it? It looks a bit like Roesel's Bush CricketMetrioptera roeseli, but not quite. The yellowish margin around the pronotum is missing (more photos here), so I don't know. Maybe not a British species?
This is a Dark Bush-cricket, Pholidoptera griseoaptera, widespread, especially in southern England and Wales, and found from northern Spain through central Europe as far north as Lapland and extending eastwards to the Caucasus.