Agriculture isn't the way to riches anymore, but holding all the former farmland is.
Say your ancestor Rich Grosvenor owned the village of Belgrave near Westminster Cathedral and the farmland around it - then just by holding on to it, watching it as it becomes prime urban real estate and living off the million-pound rents you and your children can't avoid becoming billionaires a dozen times over. Then you can proceed to buy and sell the British government and media at your pleasure, having been made a duke and a lord of the realm...
This phenomenon is exacerbated by selling leases rather than freehold rights. A leasehold may expire after 99 years, at which point it reverts to the original freeholders and can be sold again. In this manner the same land can be sold repeatedly, generation after generation, and is how the Grovenor family acquired such huge wealth.
In America, we call a version of this "redneck retirement" where a rural landowner is able to sell his property to a real estate developer when it is subsumed by urban sprawl.
Say your ancestor Rich Grosvenor owned the village of Belgrave near Westminster Cathedral and the farmland around it - then just by holding on to it, watching it as it becomes prime urban real estate and living off the million-pound rents you and your children can't avoid becoming billionaires a dozen times over. Then you can proceed to buy and sell the British government and media at your pleasure, having been made a duke and a lord of the realm...