Sarcophagid Fly
A male Sarcophaga anaces (Diptera: Sarcophagidae).
This is a species that is not included in The Sarcophagidae of Fennoscandia and Denmark, Pape, 1987 but is in Sarcophaga of France (Diptera: Sarcophagidae), Richet et al, 2011. Another such species is Sarcophaga dissimilis.
The male genitalia of S. anaces are quite distinctive:
One of four specimens taken between Kennet Mouth and Thames Valley Park, Reading, UK on 2018-09-25.