Interesting! I'm afraid I am a bit of a an invertebrate person, but the prominent groove made me wonder if it was the intertubercular groove on a humerus. I will keep scratching my head, I'm sure you will get a better answer from someone with a better idea soon.
This looks like the metatarsus of a stocky even toed ungulate (the commonest being a cow). In even toed ungulates the two metatarsals are fused into a single bone with a prominant groove down the centre.
I think this is probably part of a lower limb of a bovine animal. Possibly from a cow.
What part of the beach did you make your find?
I have recently picked up what looks like a large bovine jaw bone approximately 400mm in length. I have an earlier find of a complete lower jaw, 7 cervical vertebra, the front limbs and some ribs which also look bovine.
All of these bones were found along the Medmerry Beach on the West side of the Selsey peninsula.