The male White Wagtail, prevalent on continental Europe but a regular vagrant to Britain, looks similar to the female Pied Wagtail with black cap and bib and grey back and flanks;
the grey is lighter with olive wash rather than charcoal, the face more mottled white and the belly more shining white.
The female White Wagtail is much paler than the female Pied, with a slight mid-grey cap and a white chin; the black bib or necklace is present but much less extensive.
The female White Wagtail (above) also appears to have a thick white "supercilium" (line above the eye) caused by the grey cap and mottled lower face.
Adult male White Wagtails
White Wagtails -- Bedfordshire