@Neil_McRae
I have not used all the following, I currently use a Nest Thermostat v3 with NST Manager and apart from presence via that it is currently working for me as I have not upgraded my Nest account.
In general you cannot use separate e.g. Xiamoi temperature sensors as the smart heating system has its own.
There are Z-Wave versions of the Danfoss TRVs which could be connected directly to Smartthings, I am not sure about their thermostat.
I am planning to switch to Tado. Whilst they don’t themselves have an official Smartthings integration their product is one of the more capable ones as it supports both OpenTherm and eBus as well as the old style ‘call fo heat’. Most others do not support eBus. Vaillant and Worcester Bosch use eBus in the UK.
Warning: Vaillant upgraded their control centre module from the VR-65 to the VR-66 and Tado do not yet have compatibility with the VR-66. Worst case you have to use ‘call for heat’ but there are other possible options some of which might invalidate any Vaillant warranty.
Forget Ecobee they don’t support the UK.
There is at least one Hive integration.
There are some options for linking Drayton Wiser here.