The boiler does not...eventually there is just one super hot
bathroom, and the pipes going to it, and the boiler occasionally
firing up to keep it that way.

It is against the building regulations to install such a system. It keeps
the bathroom too hot and the central heating pipes hot when they could be
cold. Central heating pipes do lose a lot of heat.

You CAN balance everything and hope that the heat losses stay teh same,
and the master stat will do everything right, but its a darned sight
cheaper to TRV everything, so you can turn down certain rooms when not in
use.

You can TRV every room except the room thermostat one. Balance the system
properly and then turn down the room thermostat room half a turn. This
ensures that other rooms warm up quicker and the TRVs shut off before the
room thermostat is satisfied. Then, when this happens, the boiler interlock
cuts in, preventing the central heating pipes being kept warm for no reason,
which can be very wasteful, particularly on a marginal day when heating is
only really required in the morning and late evening, but is selected on all
day.

Christian.