I don’t think there is an official law in place that says they have to grant permission, but I recall itv saying they’d had the footage for a week and hadn’t released it due to police asking them not to.
Whilst the press can be bullish and insensitive at times, I’m surprised they haven’t yet printed interviews with PRs work colleagues, the neighbours, Libby’s university friends - somebody is usually happy to talk for £, sadly. So either it’s not newsworthy enough, or they’re being careful not to jeopardise the investigation. I’d be amazed if the media didn’t have much more insight from various sources.