Spotting

Hello Sam
Well let me say to start with you have chosen one of the most difficult species of British fauna to pursue,Identification wise.
Having said that I must say it's a facinating subject, especially if you live in Cornwall where you took this picture.The Isles of Scilly have some very rare species.
The best thing first of all maybe to join the British Lichen Society.They have field trips and so-on every year,visiting graveyards and various good spots.
Also you will meet people with a good knowledge of lichens willing to pass it on.
As for the best field guide.Any book by FRANK DOBSON.

These two are excellent.

Lichens: an Illustrated Guide to the British and Irish Species (6th edition) by Frank Dobson (2011). Richmond Publishing, Slough. 480 pp. The best overall field identification guide for Britain & Ireland;

The Lichens of Great Britain and Ireland edited by Clifford Smith, André Aptroot, Brian Coppins, Anthony Fletcher, Oliver Gilbert, Peter James & Pat Wolseley (2009, reprinted 2011), British Lichen Society. 1046 pp. This is the definitive laboratory identification guide with keys to 327 genera, 1873 species.

Of course on a specialist subject like this the books are expensive.
And a microscope invaluable.
Good luck.But beware lichens can become addictive.