Or have I missed the point?
Nothing missed.
Hive and the rest are intended for persons who are incapable of switching things off.
The 'features' of being able to control the heating and other devices from your phone are just novelty items which just add to the pile of things to waste time staring at a phone all day.
No one needs to switch their hot water on and off when in the park (this was in an actual Hive TV advert).
Switching the heating off automatically when the phone leaves the house can save money - but only for those who can't be bothered to set the existing controls properly, and it relies on everyone using the house to have the app installed.
Switching heating on before returning is not a feature either - heating systems do not take hours to heat the house, and if entering a building from outside it will initially seem far warmer than it is, since even with the heating off or at a low level it will still be significantly warmer than outside.
Even for those that save money on the gas, it's still a fail. This very site has many examples of people 'upgrading' from older version Hive / Nest / others, spending £100s more every couple of years to get the latest model.
None of this is automation - it's just remote control of things for little or no reason.
Smart meters are no better - the only savings there are for people who are unable to realise that leaving things switched on wastes electricity.
A separate energy monitor does the same thing at a fraction of the price of a smart meter and doesn't need someone to install it either.
The real purpose of smart meters is to enable time and demand based billing - where you pay different prices depending on the time of day and on how much power is being used. Eventually pricing on what the power is being used for - such as charging your electric car.
That will result in most people paying far more.