Largish bird, about size of a collared dove. grey back, light belly, beige chest, with speckles. Flock of 14-20.
No interactions present.
Agreeing on the basis of your description, rather than the picture, which I find hard to see clearly! A flock of such birds should only be Fieldfare, given the colours described.
My Flickr photos...
Like Ian, my agreement is based more on the description than the photo.
Robins tend to be very territorial and object when another robin invades their territory. A flock of robins seems very unlikely.
Lat/Lng: 51.747, -1.2305
OS grid ref: SP532056