Bathroom heaters don't heat instantaneously; so you either have to remember to turn it on manually (introducing your own fudge factor, as described above), or start your bathing process with a colder-than-you-want room.

> One thing I don't understand about Nest et al is - virtually no houses have actual zone cooling or heating.

UK regulations require individual rooms' temperatures to be able to be set individually. For most people, this can be done with a valve on the radiator; these valves can be "smart" and integrated into the whole "smart heating" system. For underfloor heating, this typically requires an actual wall thermostat in every room -- which is what I've got. :-)

(Hence why I also want to let the temperature in my son's room drop during the day when he's not there, but come up to 17C in the evening so he's reasonably warm under a blanket.)