Confidence: It's likely to be this, but I can't be certain.
Notes: On balance I think it is more likley to be this, as the isidia are dispersed rather than tightly grouped on pustules. Colour varies a lot in both species. Cannot comment on the C reaction, but I was testing the medulla of Melanelixia glabratula recently, and got a really good pink-red reaction. So good I decided to photograph it, but as the C reaction fades within just a few seconds, I tried to redo it on another part of physically the same specimen - no reaction at all. Tried it again on yet another part, some of the medulla reacted and some didn't. Anyhow, no question the lichen above should have reacted.