Motacilla (flava) lutea 'Yellow-headed Wagtail' isn't on the British List.
Kehoe 2006. Racial identification and assessment in Britain: a report from the RIACT subcommittee.
British Birds 99(12): 619-645.
www.bbrc.org.uk/Riact.pdf
"...occasional birds resembling the yellow-headed
M. f. lutea (which is not on the British List) are most likely to be variant
flavissima. Photographs of birds resembling these races are most welcome, but will be treated informally at present."
Best reference for ID is probably Alström & Mild 2003 (
Pipits & Wagtails).