Alyosha, You say "if other things matter so much more than BF, then why does the WHO bang on about it so much?"
The WHO speaks to governments, NGOs and other agencies, not individual mothers. They outline policies and policy changes which are, they would say, doable by these agencies which would impact on health. These policies are based on evidence. So they have loads of stuff on diabetes, obesity, nutrition - we just happen to be hearing what they say on breastfeeding in this discussion.
"Surely their time would be better spent in the UK on, for example, reducing the high still birth rate..."
They have a ton of stuff on stillbirth and neonatal death, too.
www.who.int/maternal_child_adolescent/documents/stillbirth-neonatal-death-review/en/ is just an example. Following this guidance would, say WHO, reduce all countries' stillbirth rate.
"My point is that if developed countries all did what WHO wanted to do it would have no appreciable effect on population level infant mortality, perinatal mortality, obesity, diabetes."
Yes, it would.....it would make a difference, even in developed countries, on a population level.