The two channels also have different audiences - BBC World News's audience aren't going to be happy if the schedule gets flooded with UK news, politics etc. and similarly BBC News's audience wouldn't be happy if major UK news was being supplanted by major international news - the UK floods would be down the running order for instance. The overnight solution only works because few UK viewers are watching and the UK news can be pocketed into the breaks, you couldn't do that all day. One of BBC World News's advantages is that unlike US-based international news channels it isn't that centric to its nation of origin - that shouldn't be thrown away.
A major complaint of CNN International that I've seen is that it's too US-centric. The BBC is more international despite UK Reporters and that single half-hour on weekdays. Based on the CNNI simulcasts on CNN/US, I'm sure a non-American viewer doesn't really need to know about some arrest in some US city that probably wouldn't be significant on a national level in the first place. CNNI comes off as arrogant and self-centred based on what I've seen and other people's complaints. Since BBC World News is a commercial operation, wouldn't being more UK-centric push more viewers away to for example Al Jazeera where one of their anchors said in a promo they were 'properly international'.
I do think though ultimately the BBC in the UK and the BBC outside of the UK has to serve two quite different audiences and definately feel the licence fee payer will be getting short changed by any merger. They could certainly work together more in sharing reports and programming, and that would probably lead to BBC News becoming more of an "appointment to view" service than rolling news
I get the feeling many of the international reports are already shared because many have a slight slant towards the UK but I could be wrong since the UK is a world power. And when the two news channels aren't simulcasting, many of the science, arts, and other stories on World are about or relates to Britain in some way or another.