position held" has been linked to "prime minister of the United Kingdom" 24 times in Wikidata. Illustrious predecessors of Mr Cameron like David Lloyd George or William Ewart Gladstone are missing.
This is now even more visible because of the new timelines functionality. For events to become visible on a timeline, appropriate qualifiers have to exist.
A good example is the second period when Mr Churchill was prime minister. This was from October 26, 1951 to April 7, 1955 and this makes it obvious that the current description in English for Mr Churchill is incomplete.
There are many people who have a special interest in the United Kingdom. It is for them to complete all the missing information that will show up on timelines like this one.