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M alba = Pied Wagtail and White Wagtail
M alba alba = White Wagtail
M alba yarelli = Pied Wagtail
I'm not sure whether is quite enough in this photo to be definitive as to subspecies, therefore supporting at the specific level M alba, is probably the correct action
David makes an important point, that although this is most likely to be a Pied Wagtail (yarrellii), it is best not to push the ID, if the angle is difficult and the features cannot be seen cleanly; that is why I also agreed with M. alba.
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Lat/Lng: 51.2, -0.2
OS grid ref: TQ2951