> I don't think one has to identify whom something was stolen from to know that it was stolen.
If we are talking about people, then yes, but governments are different. They are not people. Governments are law makers. So you are claiming that British law shouldn't apply and I am asking you for the name of the government whose law you think should apply. That at least deserves an answer.
The UK was ruling Egypt at the time and you have to prove its own laws weren't being followed -- which is unlikely to be the case as regardless of how we may not like Perfidious Albion, they are a stickler for things like property rights. The British museum paid for these artifacts, and you need to prove that the people who sold them those artifacts had no right to do so, but these antiquities traders were acting in accordance to British law, so you need to prove that someone else's law should have been applied instead. Whose?
> If I find something, it's not my responsibility or right to decide whose it is, and my inability to do so doesn't make it mine.
Again, people aren't governments. If you find oil in the ground, it may not belong to you, but if the UK finds oil somewhere in the mandate of Palestine, you can be sure that it belongs to the UK, since that is territory controlled by UK, and the oil belongs to the government that controls the territory, not France or China or some other government that doesn't control the territory. And if some Arab tribes conquer that region, then the oil belongs to them. And if some other nation conquers it, then the oil again gets a new owner. Or do you think the House of Saud was the first group of people to live in Saudi Arabia? No, but they were the government in charge when the oil was found, and no one has managed to conquer them -- that's the only reason it's their oil -- because they found it and they rule over the territory. And who do you think will be the owner of any artifacts that Saudi Arabia digs up as they search for more oil? Yes, it will be Saudi Arabia. The rightful owner. And they can transfer those artifacts to some museum in Riyadh if they want, even if the artifacts were made by some ancient civilization that has no genetic relationship to the house of Saud. They could be of Ethiopian origin, but Saudi Arabia is not going to "return" them to Ethiopia. This is the definition of what it means to be the government - it means your laws are the ones that apply in the territory you rule over.
I think that's the mistake you are making here. Maybe you think there was some international law in effect at the time? International law originates with the UN charter, which post-dates what the UK did, and you wouldn't want to apply a law to an action which predated the law, as that's understood to be an illegitimate application of law. If you start retroactively applying the UN charter to all government actions that happened in the past, you will find every government on earth an inveterate thief and lawbreaker - from the UK to the Egyptian Pharaohs, they all stole everything.