Fair enough, but if it's set up correctly you don't really need to
alter the timing. Although it's all up to the individual.

Unfortunately, when setting the timer in January, you don't know that a
couple of mates are coming over on July 14th until 2am. A handy nearby
"boost" button could be useful here, rather than searching for a torch
because in the low light you can't see the programmer in its cubbyhole and
can't remember which button boosts the heating (and the manufacturer decided
to scratch the legend in rather than using high contrast printing, but you
don't read braille).

2. Rather than turning off completely, the stat can just turn down
the temperature, to say, 12C, ensuring that the house doesn't get
freezing overnight, or whilst you are out.

You've lost me there, you can turn the temp up and down regardless of
where the stat and programmer are.

What I mean is that a programmable room stat doesn't turn off and on, it
just changes temperature. My "off" temperature is 10C. My "on" temperature
is 22C. If you had a separate programmer, the "off" temperature is -273C, as
it just cuts the power totally, with no ability to just keep it ticking
along for the benefit of your furniture and pipework.

So in general then to the OP, yes, your way would work but there are
other ways, a good programmable room stat would be my choice coupled
with a single channel on the water, but it's a free country!

You could always put a switch for the CH side. But then twin channel
programmers are basically the same price as single channel ones, so it would
be cheaper and neater just to use a twin channel programmer. OTOH, it isn't
that important on a single zone heating system. On multiple zones, it is far
more useful.

Christian.