I agree, I'd like more information. I'm not from the UK, but from an American point of view, they were being lost to the ocean. If the "rightful owners" wanted the items they ought to have contracted for their recovery. Then have repaid the insurance company for the claim from the loss vessel (I would think that EIC carried some form of insurance, most merchant vessels have throughout history). If they didn't know the location then it seems to be free game if they invested the time and effort to recover the items. A finders fee for recovering a cannon is a bit of an insult, IMO. The canon is a trophy for a divers hard work.
But, I'm not from the UK.