The male White Wagtail, prevalent on continental Europe but a regular vagrant to (and possibly resident in) Britain,
looks similar to the female Pied Wagtail with black cap and bib and grey back and flanks; the grey is lighter with an olive wash rather than charcoal,
the face more mottled white and the belly more shining white.
The White Wagtail with olive washed pale grey immediately above was with another in England (Cambridgeshire) in January.
I have photographed others in England in the spring. Those further above and immediately below were in southern Europe in Spring.
The female White Wagtail is much paler than the female Pied, with a slight mid-grey cap (sometimes dark) and a white chin;
the black bib or necklace is present but much less extensive.
The female White Wagtail (above centre and right) also appears to have a thick white "supercilium" (line above the eye)
caused by the grey cap and mottled lower face.
The lovely photos above of a White Wagtail feeding his young are by Dr Harsha Vardhan Reddy, used with his permission.