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.I have checked this before when replacing a thermostat on my boiler with Corgi and then Gas Safe and HSE . 'Regulation 2(6)(c) - exception for certain control devices 26 Work on any gas appliance control device which is primarily intended for use by the consumer (eg removal or replacement of an electric thermostat or an on/off timer switch) has been expressly excluded from the regulations, where this work does not involve breaking into a gasway, ie where there is no risk of gas escape, or likely interference with the safety of the gas installation.'
'expressley excluded' plus I have called Gas Safe in the past and queried this... 40 years a qualified contract electrical engineer...
I have never done work where I have touched a seal or pressurised section.
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.