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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I've only just seen this. I'm afraid I don't have the knowledge to be able to confirm your ID and your picture isn't really clear enough to tell what it might be.
If you haven't already looked there you might find the photos on http://www.lichens.lastdragon.org/ helpful.
These lichens are fiendishly difficult to ID, aren't they?!
(especially about the website, LOL, thanks Ginny)
Actually, the way the lichen is coating the trunk strongly suggests this is a Physcia species (tenella, adscendens, or very likely both) and there is a visible blackish fruitbody (apothecium) that looks just right. We do need a clearer picture, but I am reasonably confident at least of the genus, and, yes, these urban Physcia species are strong indicators of nitrogen/ammonia deposition.
Lat/Lng: 51.5404, -0.3974
OS grid ref: TQ112836