Yes, sorry if this was unclear. Some boilers have a facility to
continuously
vary their output..

Indeed, but they don't really need a continuously variable input from the
hot water cylinder. The additional benefit of knowing this information to a
control system is marginal at best.

Due to stratification, the tank doesn't so much heat up uniformly, as divide
into a hot and a cold zone, the boundary between of which moves down as
heating progresses. A single thermometer will show a very rapid move from
cold to hot as this boundary passes its installed location. This is why it
is difficult to provide good hysterysis from a single cylinder thermostat.
You actually need to sense the temperature at different heights to get a
feel for the amount of hot water available, not know the temperature at one
height.

Room thermostats are a very different matter and much useful information
could be gleaned from an analogue output sensor.

Christian.