Hi,
Can I suggest listing out your key features, and then putting them in priority order? Start with the engineering trilemma of Quality, cost, delivery (QCD) and add the things you see as important.
As it stands your requirements seem to be:
- local LAN API control (e.g. not cloud - rules out many commercial products)
- MQTT / HASS integration
- Works with UK based systems with two zones - thermostatic heating and domestic hot water.
- Unknown - suggests a hot water cylider with a thermostat, rather than a combi with instant water heating
- Unknown - Y-Plan (3-port AB valve) or S-Plan (2-port valves)?
- Unknown - CH timer + separate thermostat or combined?
- Unknown - location of key components (impacts wiring - many have a ‘wiring centre’ next to the HW tank, but some control signals are at the boiler)
- Low electronic / electrical self-build
On that basis, the closest that comes to mind is a Sonoff TH16 or Dual running Tasmota/ ESPhome plus a room temperature sensor.
- HW - likely needs only ON / OFF relay control assuming a cylinder with a mechanical thermostat.
- CH - Programmable temperature set point with ON /OFF relay control (e.g. no OpenTherm modulation)
If you add a DS18B20 to a Sonoff Dual R2 (see the Tasmota Wiki), and rewire the boiler, a single device could achieve this, BUT the temp sensor may be be located remotely next to the boiler (where the existing timer is), or next to the cylinder (where the wiring center may be).
If your boiler has separate mains level HW / CH inputs, and controls the pump itself, then a Sonoff would give you pre-packaged hardware with the option to customise the software.
Another option is to turn the existing CH mechanical thermostat down to frost (a useful backup), and use a relay in parallel as a room stat. Just set the desired temp low when you don’t need heat. A Shely One can do this, but a Sonoff would need modification to separate the relay from mains (‘dry contact’).
Sadly, most UK heating systems are wired in a more complex way to save pennies… (which also can cause issues for non-UK ‘smart’ controls). There’s lots of variation with significant differences (3-port valve, 2-port valves, separate HW, multi-zone, combined timer + thermostat, combi with integrated HW tank etc) and a picture of a generic timer sadly just isn’t enough to offer any useful advice! 
I suggest researching how the traditional Honeywell Sundial 2 or 3 (AKA Y-Plan) system skimps on components to drive water valves as usually the CH needs a change-over relay (SPCO), and all but the Sonoff 4CH Pro have only single contract relays.
There’s lots of places to start - check your existing boiler controls out, and then watch John Ward’s series on UK heating controls.
Even if you were to go out and buy a ‘simple to fit’ commercial cloud-based unit, you’ll still need to know how your existing system works - unless you pay a plumber or sparky (and they tend to know much less about the automation side, giving different issues).
James
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