This is Ochrolechia tartarea; compare another species of the same genus: 988263 (as noted there, that species is usually infertile and reproduces asexually instead).

In Scotland, from about the eighteenth century onwards, Ochrolechia tartarea was the principal ingredient used in making a red and purple dye called "cudbear" (after Cuthbert, the maiden name of the mother of the man who patented the process). These days, synthetic dyes are almost always used instead of lichen dyes; an important benefit of this change is that there is less damage to the lichen population:

"Since lichens cannot be cultivated but must be collected from the wild, and their replacement by the growth of new lichens is exceedingly slow, it is evident that the use of lichens as dyes must be discouraged because of the large quantities needed. Cultivated flowering plants should be used instead" [Jack R. Laundon, in "Lichens" (Shire Natural History)].

The presence of this species reveals something about both the prevailing climate here, and the nature of the location: this species is "common on trees, rocks and mosses in high rainfall areas, mostly in exposed upland regions" [Frank S. Dobson, in "Lichens - An illustrated guide to the British and Irish species"]. Appropriately, then, the tree stands near a footpath that is called the Upland Way: 996220.