Everyone knows someone whose cousin was at Cirencester with their best friend's sister's husband. They just do.
Own family members, 2 of my godparents... Then others at Seale Hayne (RIP), Harper Adams, (not to mention Reeseheath), etc, etc. Plus universities with big agri departments like Reading. And you're right, people do just know, in a way that you don't with people in normal higher education. The old school (college) tie is still relevant - Dad always wore his to NFU events, agricultural shows and similar. So I can recognise a Cirencester tie - I guess that's part of how people know.
My sister was at agri college with someone who'd been at Brymore the same time John Archer was scripted to be there, and said they used to call his name at registration. 