There aren't UK opt outs, The World News channel comes from Red Bee, they put in all the interstituals and programmes.
Overnight, it's effectively World that opts out of the domestic News Channel.
All of which makes it even more odd that the World News promo went out in the UK
Yes and no, Overnights are a bit of a hybrid operation. The BBC News bulletins themselves are a BBC World News production hence the program is presented from
Studio C and uses their gallery (Local time on location bugs) and the presenters are people who are on BBC World News books rather then on BBC News such as Adnam Nawaz, Mike Embly, Kasia Maderia etc. The editorial staff are also BBC World News employees and therefore is there's a conflict of interests (e.g. Mandela's death vs heavy UK storms) then BBC World's news gets priority.
However, as Inspector Sands correctly points out, control of the output lies with BBC News domestic. Hence you commonly see a snip of the domestic countdown at the end of the World Countdown as they throw to Studio C.
Red Bee look after 'opt outs' on BBC World News channel for things such as ad breaks/breakfillers or prerecorded material such as
BOTH programs, weather forecasts and features. BBC News sometimes remains with these as they don't have much else to show (Asia Business Report, Sport Today, Hard Talk etc)
Hope that makes some sense.