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Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
Quite a number of fungi form these white, rounded colonies and often they are atypical or immature.
In this case we have apothecia, which seem to be 'lecanorine', and one possibility could be Lecanora campestris - but it doesn't look right. The apothecia are too scattered, and though we don't have any idea of scale here, my feeling is they are rather small.
Another Lecanora that forms undistinguished white crusts on walls is L. albescens, but that is distinctive under a lens and has somewhat sunken, very pruinose apothecia so it is not that.
My thoughts move to a totally different lichen, Trapelia coarctata, which can be as white as this and has small, dark apothecia, but the apothecial margins tend to be more irregular or incomplete (strictly speaking they carry up part of the thallus as they develop). If the apothecia swell and turn bright pink in the rain then this becomes a possibility. The numerous, tiny, immature apothecia would appear to support this identification, but I am far from sure and will not put it up as an ID.
It isn't a Diploschistes species, which form much thicker-margined apothecia, and in any case, D. muscorum begins as a parasite of Cladonia species before it forms crusts of its own.
I looked at a population of Lecanora campestris yesterday and while most had the characteristic rather thick and slightly warted thallus, younger colonies had thin margins, as in the photograph here. Also, although most had rather densely clustered, large, mature apothecia, other colonies had more dispersed apothecia, and most colonies had the very small apothecia nearer the thallus margins that we see in the photograph.
Lat/Lng: 51.59255, -2.97531
OS grid ref: ST325886
on a wall of the park