@Arjayess
There have been two different unofficial Nest integrations for Smartthings. The first is effectively dead due to changes made by Google. The second currently works although I am wary about for how much longer it will. It is also paid for.
I agree regarding Honeywell. Their UK offerings are poor compared to the US.
Hive have just added HomeKit support to some of their products. However with regards to Smartthings here is another unofficial app
Netatmo in theory has official support as it is sort of advertised on the Smartthings site https://www.smartthings.com/gb/products/netatmo-smart-thermostat
Part of the problem is Smartthings are in the process of moving from the Classic app to the new App and it is still the case a lot of things do not work on the new App. However as you appear to have found the support is poor and it appears currently mostly broken.
Tado yet again do not provide an official app but see this.
As others have mentioned there is Drayton Wiser. Again unofficial.
Some people use basic Z-Wave room thermostat and TRVs.
There are three main standards for controlling boilers.
Call for heat (the original ‘dumb’ method) supported by everything
OpenTherm, a standard devised by Honeywell and released as an open standard and now supported by a number of brands including Ideal, Nest v3, Tado and Drayton Wiser
eBus very similar to OpenTherm but proprietary, used by Vaillant and Worcester Bosch, supported by Tado but not on the newer Vaillant VR66 controller
OpenTherm and eBus allow ‘modulated’ control which is regarded as both more efficient and delivering better more even room temperatures.
Netatmo definitely do not support hot water control. I am doubtful that Hive and Honeywell do either. Nest v3 does and so does Tado and also I believe Drayton Wiser. Although whether that aspect is supported in Smartthings is another matter.
Netatmo, Tado, Drayton Wiser all have matching TRVs.