Lungwort

This is a famous lichen: Lobaria pulmonaria. It's famous because it used to be widespread but was wiped out by air polution, to which it is very sensitive. Once gone it takes a long time to recolonise so it remains confined to areas which have always had good air quality. It is usually found on deciduous trees such as (in this case) oak and when found is considered an ancient woodland indicator. 
It used to be collected in huge quantities and used to treat diseases of the lungs (the old Doctrine of Signatures). However it was also used for tanning and even as an alternative to hops for brewing beer!