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Have you any other angles? I think it is difficult to be sure from the one photo...a view of the bird from the side or behind would help a definite specific ID.
My Flickr photos...
This one looks unusually pale to me. The second picture in particular shows an unnaturally white bird. The initial impression is definitely that of an alba White Wagtail, but I am not sure the plumage is normal. There appear, for example, to be some white feathers on the mantle in the third picture (though that could be an effect of missing feathers, showing the white feather bases below). A view of the rump would help to decide, but it does not look like a normally plumaged bird to me.
The distance that the black stretches up the rump, as far as the first tertial, might suggest that this is a Pied (or at least a hybrid), I think, as the rump is less black in White Wagtail. Just a rather oddly pale Pied Wagtail, I suspect.
Lat/Lng: 51.566, -0.043
OS grid ref: TQ357870