In the main the switched fused connection unit (left switch) is the supply to whole system so always needs to be on.
The central heating works in the main by the programmer sending power to two thermostats one for domestic hot water (DHW) the other for central heating (CH) which in turn operate a motorised valve which has micro switches inside that in turn work the boiler.
There are variations which we call plans, so C, Y, W, S for example. The C Plan uses thermal syphon for DHW often no motorised valves, (although my house C Plan and two motorised valves) the C Plan has evolved and three common versions.
The Y and W plan use a three port valve, often used with oil, or gas boilers with not cool down sequence as the boiler can cool by heating DHW.
The S Plan normally has two or more two port valves, and follows the sequence I have described to start with.
The programmer often has both electrical dip switches and physical slides to set it for the different plans, it was normally considered as a 10 or 16 option programmer depending on settings, they counted off so a bit misleading with the count.
They have gone out of popularity as the programmable thermostats starts at around £35 and instead of turning heating off, it turns it down, so really cold night heating still switches on, and I have seen the old programmer kept for DHW but wiring changed so CH is no longer controlled by it.
With so many options a picture of the programmer alone does not really help, even the wiring does not help that much as plumbers it seems are colour blind and even seen green/yellow wires used for line, where really should be only used for earth. And it seems they don't carry brown or blue sleeving as often a blue wire used for line.
The one thing we can consider as having correct colours is the motorised valve, cable fitted by the manufacturer so in the main we work back from the wiring centre where the motorised valve is fitted.
So more of a description so we know what plan is being used, and picture of wiring centre wiring, and programmer wiring etc.