Confidence: It's likely to be this, but I can't be certain.
Notes: The front edge of the eyes looks fuzzy. Unless this is a photographic effect (which I can't exclude) this would be evidence of hairy eyes, putting it in the genus Hybomitra or Atylotus [but these have more yellow hair and eyes are often pale when alive]. It is a male (eyes meet on top) - but that is as far as I - or probably anyone - dare go. They are flies of wet habitats and - at least in my area of southern Hampshire - seem quite a lot less common than the more common Tabanus species.