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
Great capture! It does look like the same bird, must have a nest nearby. Did it just stare at you or did you get scolded too?
No scolding that I noticed :)