What documents do you need to send to prove NofK status?

I would assume Birth/marriage certificates but potentially even this could prove expensive.

In the case of my wifes Great Uncle:

Birth: X4 herself, mother, Grandmother & Airman

marriage: X4 ours, mothers, Grandmothers & Great Grandparents

does this look right and is it what are required to prove NofK status?

I will apologise now if this seems a stupid question but is just occurred to me how far some would have to go back to prove kinship.

Nick