They are UK "personalities" Jeanette H - try a google search - plenty of information about them all!
Julia was the daughter of Jennifer Saunders in Absolutely Fabulous, Jeremy presents a car programme currently on SBS I think (his mum invented the Paddington Bear toy), Bill Oddie was in the Goodies, Nigella Lawson is an ex- journalist/cook/daughter of Nigel Lawson ex chancellor of the exchequer I believe.
Yes, of course they use the stars to attract the audiences - it is TV after all, and it's all about ratings.
Their past them made them who they are - same as everyone else. Many come from very humble ancestry.
Yes, a similar progarmmme finding Jo Public's ancestors would be equally as interesting.
I personally don't think it matters if you are aware of who the people are or not, in fact you may find it a more interesting programme if you've never heard of them. There has been much discussion on the boards about who people do and don't like and if you don't know who they are you see it through different, less biased eyes perhaps.