What they are doing is keeping her name in the press and in people’s minds as the longer people are talking about her, the more chance (in the parents eyes) that she will be spotted/found. I think they are, quite understandably scared, that she will become another missing child on a list somewhere. I also think that, again quite understandably, they are looking for any chink of light that might give them “something” and they have more faith in Scotland Yard of finding that than the Portuguese Police.
As for whether they know more than they are letting on, well, if I had done something I regret (admittedly not of this magnitude) I be keeping as quiet as possible not putting it into the national press.