Peter, I believe I know where you are coming from on this, I have wondered basically the same thing myself at times. I tend to think of it as whether or not there is a verifiable paper trail from the present back to those historic personages. And by verifiable I mean that the documents actually exist, transcriptions have been checked to verify that they are accurate copies, the orginals are in a records facility of some type (archives, libraries, etc) that can be checked, and hopefully all the records are usually supportable by other records. Particularly that last aspect, generally something more than casual comments by an obscure cleark that Mr. Smith was married to Miss so and so, dau of Sir such and such. I suspect tht all too often, particularly the further back in time we go, we are forced into having the obscure comment type of source as the only evidence of a relationship, resulting in a link of uncertain or low confidence level. So I believe your question concerns any whose paper trail only contains links that are verifiably certain or high confidence. And not those of modern royal descent, but rather those of us who are of ordinary descent, citizens of former British colonies.
As for my interest, many months ago I came across the relationship finder page for the MG Barons. Thought it might be interesting, I checked my relationships and found that, to my amusement, all of them were ancestors according the info returned. I checked the connections on a couple and found several uncertains in the lists, which for me means speculative. I'm sure this happens to many of us, my DNA "ethnicity" values are heavily weighted toward England and the surrounding lands so I suppose it is "possible".
Over the last five or six decades, my interest in genealogy as become more oriented toward genetic descent, particularly the last decade. And do they really expect me to believe that somewhere in those twentyfive or so generations of descent from those Barons that there wasn't a single case of someone not really being the child of their recorded father, for whatever reason?