In my limited experience of working on listed buildings, listing of
the interior was fairly unusual. *Even then, only certain parts or
aspects of the interior would be listed. *

Most listings seem to cover only the building's exterior, so *I
wondered why listed building consent would be needed for interior work
that did affected neither the structure nor the external appearance of
the building.

Bruce, listing applies to the whole property, it can't apply to just
one part or just the outside, although the local council may be more
interested in one aspect than another.
http://www.english-heritage.org.uk/s...show/nav.19378 has some
information.
Pete