I don’t know about the safety regulations in the UK, but I suspect the general issue is that then you would be 100% dependent on the SmartThings cloud being available for running your heating system. And historically it has gone out at least once a month for the last 18 months except, I think, in January 2017. Every time they push out a hub firmware update, for example, your heating system wouldn’t work.
Almost all of the other solutions work fine independently of SmartThings, SmartThings is just giving you additional scheduling options. So when smartthings is out, your heat still works.
If you just mean you intend to do this as a parallel method, people have been doing that since smartthings first existed. The original “keep me cozy“ smartapp was essentially a virtual thermostat operating off of standalone sensors.
Only you can decide what safety issues might impact your own household if the heat randomly came on or went off because of smartthings glitches, Which do happen. 
If your household consists only of able-bodied adults who can get up and Change the temperature controls manually if smartthings is acting up, that’s one thing. Personally, I definitely wouldn’t control any heating zone with SmartThings where a young child or person with disabilities slept.
Family: girl died due to heater malfunction
As I’ve mentioned before, I don’t put anything on SmartThings which wouldn’t be safe to run 24 hours unattended. But that’s just me.