Earlier quoted context omitted.
The Nest comes in two parts - the "smart" bit with the USB charger & battery & wifi, and the Heat Link - the actual radio control for your boiler. The Heat Link has a big button on which lets you turn the heating on and off, overriding what the Nest is telling it. That's not as economical as a regular thermostat, but it means you won't freeze: https://nest.com/uk/support/article/How-do-I-turn-the-heat-o...
Oh, OK, you don't have that in the US apparently. Why's it necessary?
The US version won't work with line voltage systems. It looks like the UK version only works with line voltage systems.
This also explains why in the US changing a thermostat is something pretty much anyone can do but the UK version requires a Nest Pro to hook up.
Nest US: "It's easy to install the Nest Thermostat. Everything you need comes in the box"
Nest UK: "A Nest Pro installer should install your Nest Thermostat. They will identify cables from the boiler, zone valve or junction box to connect to the Heat Link."