I would agree with you if the RS hadn’t been given to Britain via a legal treaty.
Something STOLEN by the French CANNOT be “given” to anyone else.
That is the law even in the US. You cannot steal something from someone and sell it to someone. If you buy stolen goods (whether you knew the provenence of them or not), you CANNOT keep them. That is the law in our land!
Thus an invading army cannot come to Egypt and steal and then sell it.
It would be one thing if a FREE country sold it. A great example are the Van Goghs. They are a cultural treasure of Holland but because they were sold by free will, Holland can never demand them back.
The same holds true when a country freely allows its antiquities to be sold