Here is a draft Lichen Flora of Greece (pdf). Keys to species are included.
If you use Floras from western Europe to determine collections of Greek lichens, you should proceed with caution. It has been my experience that they do not always work well. I am still unsure whether this is because there are many undescribed species in Greece, or because the full range of variation of some widespread species is not expressed in collections from western Europe. Floras from more distant geographical regions must, of course, be used with even greater circumspection.
If you would like to contribute to the effort to prepare a Greek lichen Flora, please contact me. The main need at present is for good descriptions, prepared from Greek material, for those species that do not yet have one.
Nearly 200 taxa of lichens or lichenicolous fungi have been described from Greece, and the status of many of them is unclear. You can find information on them here (.rtf file). Professional lichenologists may wish to take up the challenge of clarifying the application of some of the names.