Didn't the BBC News article state they were told by BBC about the Press Conference
Of course the BBC journalist involved will have done some research and found sources in which he was confident before publishing his article. But so will the journalists for the Radio Times and the Guardian. The BBC article isn't technically different from those, even though it comes from the same institution that will eventually confirm the finds. It's like when a newspaper owned by a big media conglomerate reports on the not-yet-confirmed plans of a TV network owned by the same conglomerate. Although the two are linked in a general sense, one is reporting news about the other, not making the official announcement.