ITV didn't know the Manchester attack was over at that point though, nor did the police.
As far as I know, there was no report that anyone said there were other terrorists attacking the Manchester arena. ITV wouldn't have had enough information to justify overnight TV coverage. Presumably, ITV believed - correctly - that there was no evidence that it was a marauding attack. Last night, evidence that there was a marauding attack in London was available relatively quickly.
ITV had more than enough information, and at a relatively early stage, that a massive news event had occurred. That they broadcast nothing, absolutely nothing, for 7 and a half hours is appalling. Whether or not it was a marauding attack is neither here nor there. They got on air last night, they should have done so for Manchester.
I do love how people with probably little or no experience in broadcast news feel they can criticise a broadcasters options.
There's so many factors, such as working hours, staff availability, resources, transport, satellite availability not to mention whether there's enough staff to crew a gallery, line up satellites / set up OSs.
That's even before you get to the actual editorial content. Who will pull clips off Facebook / twitter and load them, who will verify them and keep the programme on air .
You've also the fact more people are using mobile devices to check news sources than watch the television.
as simple as just pressing a button and you're on air.
I'd say their decision to go on last night was based on the fact they got a camera and a reporter live, otherwise you could end up with someone reading twitter and showing videos.