Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
I have not wanted to add a revision and so take away from ajm's correct identification, but this looks like f. herinii of Evernia prunastri - lacks usnic acid so is pure grey.
In any case, digital cameras often fail to reproduce the precise yellow-green of usnic acid, so identification from photos is a bit chancy.
Many thanks for your comments, much appreciated. I walk around the area where I found this specimen every week or two, so I will check the accuracy of the colour reproduction when I next pass by.
Lat/Lng: 55.7918, -4.1967
OS grid ref: NS623575