The structure appears to be laminate so this would suggest that it is horn, possibly that of an Aurochs, Bos primigenius. This ia an extinct species of wild cattle that is the ancestor of our modern domestic cattle. Bones of these, and other now extinct animals, are reasonably frequently trawled from the English Channel and hail back to the days when the land bridge existed between the UK and mainland Europe.
I agree that this looks like the horn from a species of cattle. The size being consistent with Cow rather than sheep or goat. Was this trawled or brought up by a diver? If it was by diver what wreck were they on? My wife and I have dived a few vessels with 'cargo' of animals, including mules on the Armenian and elephant tusks on the SS Maiden, both in 100m of water, neither out of Plymouth!