Depends what period you're talking about, unless there had been an overrun or unexpected event NAT always
started at 22:00:00 all through the 70s and 80s, you could set your watch by it. It was around the beginning of the 90s (ironically when ITN moved from Wells St to GIR that it started to get a bit lax)
I don't think that was the case, there are lots of examples of News at Ten on YouTube from the eighties where the clock is showing a few minutes past ten. Here's an example in appalling quality - https://youtu.be/rlVby9fHgsU
Actually in the early eighties there were actually quite a few occasions when News at Ten wasn't on at ten, I have various TV Times when on Thursdays it would quite often start at around 10.15 if they wanted to show a long film before it. Here's an example when they showed it at 10.30 so they could show the play A Voyage Round My Father - https://news.google.com/newspapers?id=6_89AAAAIBAJ&sjid=fkkMAAAAIBAJ&pg=1633%2C487597.
And sometimes if they wanted to show a long film they would do the news at nine o'clock or 9.30, I've got a TV Times where someone complains about it.
Actually one thing that did for the Nightly News was that, for the first few months at least, it wouldn't always be on at eleven o'clock and if they were showing a film that wasn't exactly two hours long, for example, the Nightly News would start at 10.50 or so, which was no good. The audience can cope with the news being on a few minutes late, but not it being ten minutes early. Although after a few months they stopped that and ensured the news started at eleven by cutting bits out of films if they were too long.
The other thing about that News at When period is that at that time, ITV were always, always running late. This was actually thanks to the deal that got News at Ten back, to compensate the ITC let them show more adverts in primetime, and hour long shows would now have three ad breaks rather than two. That meant the shows that had been made under the old structure were now longer than an hour as they had more adverts in them, and over the course of the evening ITV would end up running ages late, if you ever wanted to record anything after News at Ten you would usually have to leave the tape running for a good ten or fifteen minutes, and this carried on for about a year or so.
I still think there could have had news at 8.30 if it wasn't for Channel 5, it could have been a good bookend to keep things simple.
17.10 Evening news.
18.00 local news
18.30 ITV peak
That in the long run could have freedITV to have many great shows, including two hour dramas, or even friday night chat show at 10pm. Alas ITV mist its chance and as pointed out by Steve Williams ITV thought any old crap would do.
But the problem with the news at 8.30 is that you'd always need half hour shows for 8pm, of which there are a finite number (much like when they needed half hour shows at 10pm when the news was at 10.30), and most obviously it would interfere with so much live football, and ITV had lots of it in primetime those days. Also primetime at 6.30 was alright in the seventies and early eighties, but from the late nineties people generally came home and went to bed later.