My boiler is the Greenstar Heatslave II, an oil boiler. Relevant only in that they charge so much more for these things that they can afford to throw in a few bells and whistles. The PCB has a mains input to it (unlike older boilers which often only had a live input via the programmer) and as well as L & N available on PCB screw terminals it has a DHW and a CH input terminal, each to be connected by a programmer and/or a thermostat to the live. Their own programmer has its own private connector to the PCB and if using this links are used on the previously mentioned terminals. But it certainly has a way of enabling, or indeed not enabling, the hot water preheat. I doubt it is meaningful to demand CH and not HW, but perhaps it could this by ignoring the flow switch. Certainly when CH is off one can enable and disable the preheat by making HW available or unavailable. However, the boiler boasts an LCD panel with several buttons which can be used to set several boiler parameters, and on this there is an "ECO" button. If this button is enabled and if the DHW is also enabled then hot water is available but there is no pre-heat (assuming CH is off) and the boiler only fires when hot water flow is demanded.
The moral of the story is if you pay a 1000GBP for an oil boiler you get the controls you want, I suppose.