They're certainly not the only people capable of tracing it back. They might be the only people capable of tracing it back correctly :-)

I'm fairly sure you only get back beyond a certain point by finding a handy noble in your tree and it's the "finding a handy noble" bit where a lot of the wish-fulfillment comes in. The Benthalls would appear to be my handy noble.

Conventional wisdom in genealogy further states that any one who has traced their lineage back to a character in the bible is clearly mad. The Windsors, incidentally, can do this (if you allow medieval genealogies as evidence).