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I agree it is ssp. pica (only because the literature says it has to be!).
There is not really much point in including subspecies for a bird like Magpie, though, as the differences between them are so minute as to be impossible to note in the field. If the photo was to point out a difference between ssp., it would be worth doing.
It's different in something like Pied and White Wagtail, where the subspecies are clearly different.
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Subspecies of birds like Magpie are not assignable with the naked eye, unless in the hand and being measured very carefully. Go out and catch some and measure them and I will believe it's pica, but don't make assumptions, I would say. Who knows whether ssp galliae strays across the channel, occasionally? We shall never know if we don't keep an open mind. For me, then, it is just Pica pica, really, unless the observer can prove otherwise.
Lat/Lng: 51.0783, -0.3355
OS grid ref: TQ166323