Geographic variation and taxonomy, very complex. Nine to eleven subspecies currently recognized, depending on the authority. This is the British race, sometime considered a separate species under the name "Pied Wagtail." It breeds primarily in the British Isles and adjacent Europe. It is characterized by a mostly black back in adult males such as this. Also note the extensive gray sides and flanks. Nikon P510 CoolPix.
References:
Adriaens, P., Bosman D. & Elst, J. (2010) White Wagtail and Pied Wagtail: a new look. Dutch Birding 32: 229-250.
Alström, P. and K. Mild. 2003. Pipits and wagtails. Princeton: Princeton University Press.
Badyaev, A. V., D. D. Gibson, B. Kessel, P. Pyle, and M. A. Patten (2017). White Wagtail (Motacilla alba), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. Retrieved from Birds of North America: https://birdsna.org/Species-Account/bna/species/whiwag
Morlan, J. 1981. Status and identification of forms of White Wagtails in western North America. Continental Birdlife no. 2:37-50.
Tyler, S. (2018). White Wagtail (Motacilla alba). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/57821 on 9 September 2018).