I think this is a (probably young) Cetti’s Warbler, which is a first time photo for me. They are certainly found in that area (Reading, UK), and Merlin Bird Photo ID points to it for the following photo taken at the same time and place:
Canon R6 + RF800mm
He’s doing the Jean-Claude Van Damme pose.
Jean-Claude Van Warbler?
(I’ll see myself out)
Super athletic tbh