I don’t have anything deep or profound to note about the program Law & Order: UK, but each time I watch it, I see Harriet Walter as Harriet Vane of the Lord Peter Wimsey adaptations (and more recently as Honoria Bulstrode in Christie’s Poirot story of Cat Among the Pigeons) , and now Peter Davison as Tristan Farnon in All Creatures Great and Small. I missed Davison’s stint as Dr. Who, so I don’t have that memory.
These connections just point up for me the narrowness of the number of actors at the top.
Update: And now, watching Person of Interest, I see Ruben Santiago-Hudson (Capt. Roy Montgomery) late of Castle. It makes my head spin.