Iichen on apple tree

Lichen will not do your tree any harm at all. As you said it is a result of the cleaner air (less sulphur) that we are getting now.
There are a few diseases that can mimic lichen on tree bark but will also be found on the leaves,buds and fruit. These are the ones to worry about as they will reduce your fruit yield and damage the tree

If you really must get rid of it (Sometimes things like woolly aphids can overwinter in it) then the simplest way is to wash it off. On a large tree use a power spray, set so that the strength does not damage the bark of the tree.
There are organic alternatives to Winter Tar oil wash by the way.

But lichen is beautiful! And i suspect it helps insulate the tree from the extremes of the weather as well as sheltering beneficial insects.

When I visited Kellie Castle recently, I saw that the apple trees were almost completely covered with silvery-grey lichen, and looked great (my pics didn't come out great, sadly). I would think, therefore, that if the National Trust doesn't remove this from the trees in this organically-managed garden, it can't be damaging.