Even within each sub-brand of the group, they often work with different manufacturers.
Though Sytner (the biggest) tend to have single-manufacturer dealerships.
Probably a mix of both on both sides of the pond I imagine?
And there's less rigmarole during the process. Less aggressive sales tactics I believe
Often there will be multiple dealers on adjoining lots, owned by the same conglomerate- but they'll maintain some illusion of independence.
I bought a Hyundai recently and the Hyundai dealer was right next to a Volvo dealer and a VW dealer. They're all part of "Sheppard Auto Group" and they share a parking lot, but the buildings are completely disconnected. However when it came time to actually sign the paperwork they led me from the Hyundai dealer to an office in the VW building, because that's where the sales manager who was working that day was. They also share a service department.
However, if I'd wanted to buy a VW though and I went in to the Hyundai building I suspect they would've made me walk over to the VW building and talk to those salespeople, and all my paperwork has the name of the Hyundai dealer on it. The point is you'd never go to "Sam's Car Dealership" and find a Ford parked next to a Honda parked next to a Chevrolet.
Independent used car lots are a free-for-all though.
