The Cladonia Key is available in two forms, for Windows PCs and Mac, so select the one you need. You will also need the instructions. Any feedback on this key should be sent to Neil Sanderson at email and Les Knight at email. It is much in need of updating to the new Cladoniaceae volume 26 of Revisions of British and Irish Lichens, which can be downloaded from here. Neil Sanderson intends to have go at up dating when he finds the time, but in the mean time using the species accounts in this website should help link.
In addition, Neil has written a "A Guide to English Cladonia Species and Associated Lichens", which can be downloaded below. This uses an informal classification by morphotype and links to the species accounts in this website. This will be updated to cover all of Britain at some point.