Myth alert. There are lots of Victorians with cavity walls. Half inch
cavity was a popular standard. The cavity was to reduce dampness, not
for insulation, so half inch was all that was needed. There are lots
of solid wall Vics, but also many cavity wall ones too.

Yes, my parent's 1890s house is very unusual. There is a different
construction on every floor.

lower ground floor: double brick, cavity, single brick
upper ground floor: triple brick
1st floor: double brick
2nd floor: single brick

At the floor level on every set of stairs, there is either a "ramp" or a
useful shelf as the wall gets thinner. I strongly suspect the lower ground
floor cavity is due to the fact that this floor is half buried in the
hillside.

Christian.