Boiler interlock is achieved by the correct use of room thermostat(s) or programmable room thermostat(s), the cylinder thermostat and zone valve(s) in conjunction with the timing devices. These should be wired such that when there is no demand from the heating or hot water both the pump and boiler are switched off. The use of TRV's alone does not provide interlock.

I'm not an expert but in older systems IIRC, the decision as to whether the boiler would fire up or not was simply determined by the boiler thermostat whether or not there was any actual demand for heat from either the CH or HW.

When I designed my system 16 years ago, I used separate motorised valves for HW and CH. The pump and boiler would only turn on when either valve was open. Whether the valve was open depended on whether the HW cylinder stat was demanding heat or if the room thermostat was after heat.  Probably much more common nowadays but fairly rare then.  But I didn't get the plumbing part quite right..see my post on pumping over