A male Musca autumnalis (Diptera: Muscidae).
The males of M. domestica are similar but on the top of the head the gap between their eyes is larger. While I have probably seen several hundred M. autumnalis over the past few years, I have not yet seen a single fly that was recognizably M. domestica.
This picture more clearly shows the orange sides of the abdomen:
Photos taken in Whiteknights Park, Reading, UK, on 2013-08-03.