Yeah I suppose the pumps are small and don’t mind if there’s a large flow (all underfloor legs open) or only a small flow from 1 underfloor leg (worst case). On large 2 port systems the pump speed matches the system flow, as the valves shut the pump slows down, older systems were 3 port with a bypass leg with a balance valve that matched the heater coil resistance!
EDIT:
As long as one UFH circuit is open, the inline TRV on the flow side can still be closed and there would be no dead end.
I was kind of thinking from the boiler primary pump if all the TRV’s are closed.