A robber fly (family Asilidae, order Diptera), probably Leptogaster cylindrica (striped slender robber fly). Michael Chinery (Complete British Insects, Collins, 2005) mentions Leptogaster guttiventris (dashed slender robber fly) as being similar, but comparison with the photos here and here at Morphbank gives a better match to L. cylindrica.
Photo taken in Reading University grounds, Reading, UK, on 2009-06-21.
I can now confirm that this is Leptogaster cylindrica, and not L. guttiventris. The above image clearly shows a dark grey longitudinal stripe running down the dorsal surface of (at least) the last 3 abdominal segments. According to British Soldierflies and their Allies (Stubbs and Drake, 2001) this band is only continuous in L. cyindrica. In L. guttiventris the band is broken at the posterior end of each segment.