There’s a quite inexpensive thermostat and boiler controller set (two devices) available at Vesternet that uses Z wave, so it joins directly to the smartthings hub, no additional bridge or cloud required. It’s very basic, just a simple thermostat and boiler control, but there are quite a few UK members using it. Cost for the set is typically about £85. 
There’s a DTH and discussion in the following thread (this is a clickable link) if you’d like to talk to people who are actually using it:
@brumster might have more to add.
( note that although there was a platform problem about two years ago that did cause the integration to stop working for a while, that has since been fixed. See the later post in the thread. )