Probably so. I imagine the U.K. has laws that make such artifacts the formal property of the state. Was this true for 19th century Egypt?

I think it was true, on some level, at the time the bust of Nefertiti was taken in the 1920s. Supposedly, the Germans nominally followed some sort of legal process for removing the artifact -- though perhaps with less-than-full transparency.

Perhaps there are other reasons to claim that Egyptian artifacts were 'stolen.' But I'm trying to have a conversation about what those might be since the subject is not as obvious to me as others seem to think it is.

It seems likely that designating artifacts property of the state is something that happened as a result of looting/stealing/collecting in the past.

We see this happening now on a smaller scale with metal detecting and what happens with their finds.