In Britain primogeniture wasn't just a cultural norm, it was the law. You also had entailment to help prevent property leaving the family. Army commissions were an expensive proposition while parishes tended to be in the de facto or de jure disposition of local landowners, along with the income that came with them. Both systems, along with the legal and civil service professions (the other two acceptable careers for noble and gentry sons) provided income and preserved control over the levers of power in the hands of the landed classes. To further help things the British elites routinely incorporated especially successful commoners - not just by ennobling, but by giving or requiring the acquisition of land. That had the effect of limiting bourgeois discontent while making sure that the economic, social, and political interests of the newcomers were aligned with the old elites.

It was a slick system, and it only really began to break down in the last third of the nineteenth century courtesy of expanded suffrage and agricultural depression.