To be honest, a heating system is a really expensive and dangerous equipment to skrew it up.
If you overheat the system, it can explode. If you do not turn it on when the temperature drops below 0 Celsius, then you can burst the pipes.
Generally SmartThings as a cloud based system, it is not reliable enough to handle this kind of scenarios. (Just look at the status page how many outages has happened during the past few months.) You rely on SmartThings cloud solution at the first level, then you rely on your network equipment, and then your internet provider, and last but not least on your electricity provider as well (just think of short power outages). It is definitely not reliable enough to control a heating system, which requires a failsafe.
I know it is the most expensive solution, but it is indeed to have the right controller connected to your boiler. One which can handle your central heating boiler and your electric towel rail at the same time. I just guess, but when you built/bought your house and the electric towel rail was fitted, the idea was to get the towel dry during spring and autumn, when you do not use the central heating. So, you physically switch / plug it to be on or off. It wasn’t really designed for mixed mode I guess. If it was, then you should have a wire running from the towel rail’s electric heater to your boiler or to your central heating controller thermostat. Because that’s where the control should be centralized.
I have a heating system, which consist a gas boiler, a circuit of radiators, a circuit of underfloor heating, a circuit of domestic hot water, and another heat source, a fireplace with a built in water tank.
The gas boiler is controlled by Viesmann controller, I cannot remember which model, and the fire place has a temperature sensor in the chimey/or on the water tank, I am not sure, which reads the temperature. The temperature sensor is connected to a controller, which turns some pumps on the selected circuits and forces the boiler to shut off when the fireplace is on. All wired from the fireplace to the cellar, where the boiler and pumps are. That has a few level of failsafes, but I can still overheat the water in the circuit, if I put too much wood on the fireplace. And to be honest, I would never think to put it on any automation system, which depends on the internet or any overcomplicated solution.
Just my 2 cents…