Don't know much about maritime laws. In firm ground, if you find something and want to go the lawful way, you declare it and it would be announced to find the legitimate owner. If he doesn't appear, after a given time it's yours. If he appears, I think you have the right to a reward (a percentage of the value). The same applies to treasures and the like.
I saw the images of the people pushing the beemer out of the crate. It's in my opinion a poisonous gift. You have the bike but you can't title it because you don't have proof of ownership nor have you paid the import duties. Much better to contact UK BMW and ask for a reward. I'm pretty sure BMW would be more than happy to pay 15% of the bike and pick it up by its own means, or at least the insurance will be happy with that. a 10% of a tourer BMW makes for a hefty sum...
Raul