Ummm! 
Some fall on stoney ground! 
Threads on RC often get sidetracked

To get back to your original point - pre 1837 it's (in England & Wales) very much a matter of checking and rechecking the Parish registers, Grave references etc and often following a side branch just to provide that piece of confirmation needed.
If you are really lucky you may find a link to what is known as a "Gateway Ancestor" - generally from one of the Nobility Houses whose genealogical lines have been preserved - although again the watchword is , as always, caution. Some medieval histories are honest endeavours while others may make wild and unsubstantiated claims.
The references regarding Mathew Pinsent on the TV show referred to a gateway ancestor of his whose lineage was traced back through several high ranking families to the Royal Family. The original medieval researcher to show that the king was governing by divine right somehow managed to trace a link to Christ.