This striking yellow and black fly looked to me like a wasp-impersonating hoverfly when I first saw it but it turned out to be neither! It is just a fly despite that bold appearance. It is one of the family called soldier flies because of the smart uniform they wear. This species was recently given the common name of the banded general (Stratiomys potamida). Its larvae feed on algae and rotting vegetable matter in very damp areas so you will often find the fly itself in similar habitat, either on the ground laying eggs or perhaps nectaring on nearby umbellifer flowers such as Hemlock-water Dropwort, Hogweed and Angelica.
Seen from June until early September this certainly a species of southern England but it is not common. My reference book, "Insects of Britain and Ireland" by Paul Brock suggests that it has been becoming more frequent since the 1970's and it will be interesting to see how it fairs given the general decline in insects in recent years.
One of three similar species so one needs a careful eye to distinguish which one, the 'eye' that helped me with this one was the amazing I-Spot website!