Very good question and apologise for being late on chat. Lichens can be found throughout the UK but the finest examples are to be found in areas that are not disturbed by human activity or come to that, pollution such as car fumes. If I wished to find the greatest variety of Lichens on trees I would look to the National Parks which are the most ‘wild’ as well as the Moors and Dales (ie. Bodmin Moor, Dartmoor, Yorkshire Dales, Lake District, Peak District, Cotswolds).
Some Tree Lichens can grow on any tree or sometimes preferentially deciduous or evergreen. Others are almost species specific so have been put on the endangered list. These include the beautiful Golden Hair lichen (see http://www.the-lizard.org/index.php/article-archives/lizard-flora/318-golden-hair-lichen ). The same fate could be in store for those Lichens that grow specifically on Ash trees as they are being threatened by a another fungal species, almost certainly imported on diseased trees. Ironically, the same high pH of Ash bark meant that it was the last natural refuge of Lichens that grew so well on Dutch Elms..Please vote for Lichens!