I would imagine diplomats or other officials from countries/organisations that both the UK/Russia can have faith in. The alleged incident occurred on March 4th. As I've bored myself stating the same thing, I'll let you construct a scenario for why both father and daughter haven't wanted to use phones and why they haven't had their photos or moving images taken to, if nothing else, reassure people they care about that they're content. And also to spike Russian accusations.