Large trifoliate leaves and pale purple flowers with green centre.
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Oxalis acetosella does throw up specimens with pinkish or purplish flowers. Why do you think it's Oxalis violacea, which may never have been recorded from the wild in Britain (not in Stace; not on BSBI mapping website).
(A review in Watsonia 4: 51-59 (1957) states that all records of Oxalis violacea in Britian were errors for Oxalis articulata and Oxalis corymbosa - on the other hand that is over 50 years old.)
There are varietal names for colour forms of Oxalis acetosella - the above cited review gives rosea, purpurascens, subpurpurascens, violacea and caerulea.
A very good question. Unfortunately I do not have access to the learned tomes mentioned and rely on searching my library of wildflower books followed by searches of images on other websites.
acetosella shows the normal white forms with a couple of all pink versions.
articulata seems to be all pink with a dark centre.
corymbosa seem very variable and do show some flowers with green centres but petal shape different.
violacea has identical photos to my spot, hence my original i.d.
As always, I am happy to be put right and welcome comments from other sorrel experts.
I've had a closer look at your photograph, and agree that it is one of the pink-sorrels, rather than Oxalis acetosella.
Up here they are all rare casuals (associated with garden waste), but some are established in the wild in your neck of the woods. Oxalis articulata is the commonest, followed by Oxalis latifolia) and Oxalis deppei (=corymbosa). I don't know these plants, but the "obsagittate" leaves suggest Oxalis latifolia. (Fide an image search, the flowers seem to match as well.)
Lat/Lng: 50.4814, -3.5158
OS grid ref: SX925657