I'm not sure what it's got in its beak in the second picture.
No interactions present.
Reed is a browner bird, with a rounded tail and a longer bill on an angular head...no green tones at all. Very different warblers.
My Flickr photos...
I think it might be a faecal sac, if it is a parent attending young.
Lat/Lng: 51.77357, 0.00864
OS grid ref: TL386102