The old insulation should have been removed. This would make the
loft bedroom much warmer, probably enough to need only very minimal
heating, if any. Is it impossible to remove it now? This seems like
a lot less work than all the heating extensions you are planning.

I disagree. If the OP intends to have subzoned heating, then insulation
between zones is actually quite useful, as heat required for occupied rooms
doesn't leak into unoccupied zones.

Christian.