Hi, and welcome to the UK Fungi website.
Not Hydnellum peckii, as that one grows in coniferous forest soils, always associated with Pine trees. (In the UK, its distribution is confined to the Scottish Highlands).
If the tree was definitely Willow, this will more than likely be a freshly emerged fruitbody of “Willow Bracket” – Phellinus igniarius.
The species does also appear on other deciduous trees, and is not particularly common in the UK.
As the fruitbody matures, it will grow much larger, becoming very “woody” and “hoof” shaped with a grey cap, and probably cracking with age.
Regards,
Mike.