Typical wiring
You can see the wiring diagram in the main some where also there would be a thermostat or two, they do not offer central heating and no domestic hot water as used with old C plan, the old electronic units had options to switch off the CH only both mechanical and electrical but latter ones did not, so you end up with a unit which says you can have just CH but you can't in fact get it. They will work, but you can't turn DHW off.
So you may want to make other changes, so you need to plan the final aim so what you do on the way, will not stop you getting the final goal.
The original C Plan changes between DHW and CH + DHW by switching the pump on/off, later versions used a thermostat on the hot water tank so in summer you could regulate DHW temperature (although not in winter) and still latter a motorised valve was added so DHW temperature controlled summer and winter, and also you could have CH only, the latter does depend on boiler, some boilers need DHW always connected to cool down.
I found with mine the CH upstairs would still get warm even when off, and had to add a motorised valve to ensure in summer it stays off, as a result of fitting motorised valves the place where the control board is located also changed, hence why I say decide what the final goal it first.
The whole idea of early central heating was to get up in a warm house and return home to a warm house, there was still an open fire for normal heating, I really don't want an open fire, so I needed to increase control, but you need to decide how much you want to increase control, using Drayton Wiser or EvoHome you can have each room independently controlled, but you can get a programmable thermostat for £35, so you need to decide if your willing to spend £20 or £500 to increase the control, and if your going to do it all together or bit at a time.
I fitted Nest Gen 3 not because I wanted to control with my phone, but I found in that nest of wires, there was only two wires going to boiler, and with Nest Gen 3 I could control both CH and DHW with two wires, my wiring down stairs in what is now a flat and was a garage became complex. This is how the flat looks now
there were far too many wires to fit in the Nest heat link so I used a wiring centre below it, in hind sight Nest was not the best option, but hind sight is easy.
So step one work out what you have, photos likely the best option, once you know what you have, then work out the final aim, and the route to get there.
