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It is yarrellii, because of the black back, which alba lacks. The latitude is nothing to do with the identification, as both are common here at different times.
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The basis of a Sub Sp is local adaption The link says it all
Your point about the variation in species is seen in the link above.
It is correct to refer to the dominant Sub Spp at this Latitude as subsp. yarrellii you are also correct to point out that there may be some overlap
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During migration there are some locations in Britain where the dominant subspecies is M alba alba (White Wagtail), so it could be best to qualify such statements, by referring to the breeding distribution.
The issue of latitude is not important, either, since alba is found both north and south of the UK, and level with it in continental Europe.
Lat/Lng: 50.718, -1.744
OS grid ref: SZ181909