Motacilla alba
Appearance
Black, white and grey plumage
Black throat and crown
Black, white and grey plumage
Black throat and crown
White face
So, apparently there are nine subspecies of White Wagtail. The Moticilla alba yarrellii (Pied Wagtail) is the most common in the UK. However, the Motacilla alba alba (White Wagtail), which is found in continental Europe, migrates through Britain from early March to May and again from mid-August to early November. Your guess is as good as mine which one this is. I saw it in Thame, Oxfordshire today.
Learn more about the White Wagtail.