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.
Certainly Evernia prunastri, though not the healthiest material I have seen. There is a pronounced algal layer on the top surface, not part of the lichen.
EDIT (24.3.2011): Sorry, I looked at this too quickly. It has well-defined discoid soralia along the edges of the lobes. No wonder it looks "unhealthy" for Evernia - it is Ramalina farinacea!
(Of course, it looks unhealthy for that too ...)
Thanks for the comments, it's interesting that the algae layer is so predominant, quite a few were like this and I thought it may be something to do with reproduction!
Lat/Lng: 52.1, -3.4
OS grid ref: SO0451