I'm sorry to say the protruding white fungal growths look like small versions of Crepidotus, a small bracket fungus which grows on dead wood and helps to break it down. The white growths you can see are only the fruiting bodies of mycelium already at work inside the tree. Its presence indicates there's a problem within the tree, and its wise to call a reputable tree surgeon to check whether the tree is stable and safe, and if not, have it removed.

It means that at least part of the living, outer part of the trunk has died, leaving an entry point for Crepidotus and other wood destroyers to reach the supporting--though dead--heartwood. The tree may survive for some years more, but it will become hollow, and easy to blow down in a storm.

THANKS FOR YOUR HELP

The bark looks more like an oak tree to me. Silver birch bark is smoother and silvery white.

added new photo pretty sure its silver birch

Yes its a silver birch.

Yep. Silver birch bark gets rougher and darker as the tree ages.

Yes but it was very difficult to see on the first picture.

they are not that long lived according to the RHS advice I got. our 3 were clearly dying and were 50-55yrs old and I was told that was a good age for them. So I suspect yours is very mature as this is what mine looked liked. they are one of the species that moves into the pre-climax stage of succession of plants from bare earth to full oak forest.

If you happen to be into fungi there's a grand bracket fungus that grows only on dying birches that used to be used for stropping razors on. (Not that its anything to do with the question but perhaps you might be interested...)