Using Rob's visualisation of the Open UPRN data: https://osm.mathmos.net/addresses/uprn/

It's rather manual, though. I have Rob's tools open in one tab and iD editor in the other, and copy across UPRNs where there is a single UPRN for a building and the centroid exactly matches. I always find on a survey that some buildings haven't been split yet, so I use StreetComplete to make the house number something like "7,9" and edit the outlines later. The UPRN tool found some places where I'd forgotten the "later" bit :-)