They want to make sure the race of the attacker is not revealed.
You can be sure if it were a white that did the stabbing, the police would be asking for video and the perp's photo would already be plastered all over the international news.
They are asking for videos - see post 5 above.
It's routine for UK police to ask for videos of a crime scene not to be posted on social media, as widespread circulation could be used by the defence in the future trial to claim jury prejudice. The legal rules on this are strict in the UK.