Pied Wagtail in foreground. White Wagtail in back????
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As Chris says, both are Pied, with dusky flanks and black/dark grey backs. White would have cleaner white flanks and pale grey back.
Wagtails undergo a partial winter moult, as well as the usual autumn moult, and have different plumages for male/female. Add to that first winter plumage...etc and that leads to variety...which you can see here. This could be a pair, I suppose.
My Flickr photos...
Lat/Lng: 51.5647, -0.0411
OS grid ref: TQ358869