Unknown Fungi

Had a short very wet walk this morning. Much better this afternoon. I'd passed this fungi a few times, but decided to photograph it today. There were several very large brackets all 30cm plus across as well as some very young ones with the same white top. The bottom image shows one where a part has been broken off. You can see it has a very deep pore layer. I think the tree was Oak, but as it was pretty dead I'm not 100% sure. So if anybody knows what it is that would be great.