Although a difficult picture, because I have the feeling that colurs are not "real" due to harsh light and haze, I think its a Garden Warbler.
Bill shape is very good for recently fletched Garden Warbler (and wrong for similar aged Blackcaps, with the caveat that growning bill can show unusual shapes).
Also in many young Blackcaps the cap is already appearent, WHEN You Know IT MUST BE THERE or clearly visible. In this bird this cap isnt there, you can look at this picture for one more day ... (I hope thats understandable)
Legs are pale with a flesh tinge, but thats ok for many juvenile passerines, regardless of adult or first winter leg colour.
A quick google search produced this:
https://parrotletsuk.typepad.com/wl...-devon-bird-ringing-and-a-garden-warbler.html
Convinced?