What is this rock? UK origin, non metallic, heavy
Your rock is most likely a marcasite nodule. These nodules weather out of the chalk beds along the coast.
Here is a reference to UK chalk beds.
If you are interested, I would like to trade for one. I'll trade you a nice pyrite nodule I found here in northern Ohio near Lake Erie.
See this similar question: What are these metallic nodules that were found at low tide on the UK south coast?
Not coal... coal is very low density, so wouldn't be "heavy."
Looks like an ironstone concretion. It looks suspiciously like a sectarian nodule (google it), but the only way to tell that would be to cut it open with a rock saw. (This would be a helpful thing to do, anyway.)