Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
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It is difficult to make out details with the images on my screen. If gravestone is limestone then another possibilty might be Dirina massiliensis f. sorediata.
Higher resolution images on the way!
Good to see pictures in larger size, but unfortunately some of these white things on gravestones and church walls really do need very sharp focus (and are best not photographed with a strong blue colour cast). Like another of your observations I have copied images into Photoshop and done my best to correct the colour, but not to much success.
I am fairly sure it is not Caloplaca teicholyta, the margin is wrong. As Phil says, Dirina is a possibility - but best to check it in the field by scratching it and seeing if the scratches are yellow. Lecanora pannonica can be very like Dirina but does not scratch yellow. They are not easy.
Lat/Lng: 51.0484, -0.7219
OS grid ref: SU896284