I'm not going to start making this a moral issue, but on a technical note, my understanding is that since modern boilers are effectively computer controlled, a simple thermostat is probably not that simple unless it's a [*** xxxxxxx xxxxxxxx xxxxxx - if you know what I mean: good; if not: never mind] in which case, it actually is that simple. If you look at the examples you have given they are about the user interface, not about sensors that may well allow the PCB to interpret how best to regulate the boiler operation, so I'm not convinced you would be on the right side of the law.
Point is, people who are trained in safety issues tend to be serious about it, and, this being a public forum, ANYONE could read this. So regardless of how many years' experience you have as an electrical engineer, you are unlikely to get much help on this forum for the reason that your neighbour, who is not, might well read this forum and also have a go.