In my Boler - and my case it not unique - the white wires come together somewhere in the nest of random wiring and connect to each other with a variety of crimped-on wiring caps (circles in my diagram, below). There is no bus bar anywhere for the negative side.
is wired something like that but the white wire eventually comes back to the (-) terminal on the fuse block board. This can be seen in the Scamp
Wiring Diagram in Helpful Documents.
Any circuits I added have a 'home-run' negative lead as well as the fused positive lead, just as done in 120VAC wiring -- Easy enough to run the negative wire when I'm running the fused positive one. I'd rather have a few extra wires with connections only on the ends than have connections buried in the runs -- Makes for less trouble, easier troubleshooting and no worries about resistance loss.
If I were wiring from scratch, I would have every internal circuit set up with two wires on a fuse block like the one in the post above and would probably have terminal strips for external lighting
as well to increase reliability and aid troubleshooting. However, I can understand why manufacturers wouldn't go for the extra equipment and labor.