With no crime committed You have to have the lawsuit filed
Wrong.
(At least in the USA. Dunno about the UK.)
Unlawful Conversion is a strict liability crime.
The only fact that matters is: did you do the act that resulted in the unlawful conversion. Doesn't matter why, or what you were thinking.
Sega can be charged criminally first, based on the fact that you knowingly converted the property. The criminal intent element of Unlawful Conversion does not require mens rea, only that the defendant exercised unauthorized dominion or control over the plaintiff's property and intended to do the act that interfered with the property owner's rights. It does not matter if this was due to a Mistake (such as ignorance of what they were doing).
And if the ultimate receiver does not return the property upon request, they can also be criminally charged. (They can try suing the party that sold it to them. But not knowing that they were acquiring stolen goods, and having acting in good faith to buy it, does not mean they get to keep it. If they don't hand it right over, that's a crime. "Sorry, buddy, you got fucked.")
The law is NOT "Finders Keepers".
Also, this is not analogous to someone throwing out (or donating) something accidentally. Because in those cases, the person disposing of the item was in lawful possession and had the right to mistakenly get rid of it. And so maybe (probably) the receiver gets to keep it. Whereas in this case, Sega disposed of property that they were leasing from Nintendo, which they had no right to dispose of. They basically stole it.
I think Sega can go after the possessors, since Sega is still under contract to posses and otherwise take care of the property. But Nintendo as the actual owner could come along and demand it back as well, from whoever has it at the moment.
As I said, I don't know anything about UK law; this is just how it works in the US. But assuming it's the same, the police were just acting properly under appropriate warrants, and criminal charges are certainly available.
There could also be civil matters, but in the first place, this is a criminal matter. Whether the government wants to prosecute it is (as always) a question. But "criminal intent" is not a factor. It's a crime regardless of any intent, lack of intent, mistake, ignorance, etc. Sega did the deed - it's as simple as that. Since that is easily proved, open and shut case with no possible defense.