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I don't think that you can go much further with confidence, though something like Sedge Warbler is my best guess. Sometimes, a photo just isn't good enough to get to the specific ID, sometimes even genus is difficult.
Did you get any other angles?
My Flickr photos...
ive uploaded two more photographs
It looks good for a Cetti's warbler, though they normally skulk more than this. Did you hear an incredibly explosive, metallic, repetitive song of 'chutt' like sounds? Normally, if a Cetti's is around, your eardrums know about it!
Cetti's Warbler was my identification when I saw the thumbnail, and the additional photos leave me with absolutely no doubt.
Lat/Lng: 51.1541, -2.7782
OS grid ref: ST456397