Adidat thought Lignum, I can see where he's coming from. Another possibility might have been Bocote.
This is the original Lignum on the left (unobtainable now) and Bocote on the right.
When you work Lignum there's a very distinctive smell, sort of refreshing but slightly medicinal, and the waxiness of the surface is a dead give away, not even the very best Rosewoods have that degree of oiliness. And of course they're both very heavy, Lignum slightly more so (it sinks in fresh water) but you'd be heard pressed to tell the difference simply by hefting a lump.
But it's difficult to see how or why either of those two timbers would ever be kicking around a British beach!
Greenheart is an interesting one. Given it was used in massive quantities all around the coast for groynes, piers, docks, and other marine installations it's easy to see why some might fetch up abandoned. But your sample looks more variated in colour than the Greenheart I've seen. A couple of years ago a pier was restored in the town where I live and I bought up a load of the salvaged Greenheart. I turned it into garden furniture and it sold well locally given its history. Unfortunately it's all gone now but I don't remember those warmer tones in the wood, it's called "Greenheart" for a good reason!
Hey, it's all a guessing game, so your "Mystery Wood" label is as good a name as any and more honest than most!
Good luck!
