My son Sam found this on the beach. He identified it as an ammonite as he had heard they look 'like an ice cream sandwich'. You can see a bit of what is inside in the picture.
Not solved at all, I think. I don't believe you'll be finding Jaekelopterus fossils in Britain. They have a rather limited geographical distribution in the fossil record, for which the nearest known locations to Britain are in Germany. Even other Eurypterid fossils would be pretty unlikely finds on our South Coast, although we do have strata of the correct age.
There aren't really enough diagnostic features visible in your picture for a reliable identification.
Eurypterids first appeared during the Early Ordovician or Late Cambrian period (c467.3 million years ago) and disappeared before or during the Permian-Triassic extinction event (c251.9 million years ago). Although we have deposits spanning those time periods on our south coast, they won’t be yielding Eurypterids… and certainly not in 3D preservation.
This is the only British Eurypterid fossil I have (Erettopterus [Pterygotus] bilobus), sadly missing its head and claws. It’s from Silurian deposits in Scotland from around 430 million years ago and, again, the nature of the strata in Scotland is they don’t yield such fossils in 3D preservation.