We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Andy Dingley
saying something like:

So what are my pros and cons for adding insulation to either the
inside or outside of the wall?

Where I've seen external insulation applied it's been quite effective,
but it depends on attention to detail to make it work properly.
The advantages are mainly; saving of floor space internally, very little
internal disruption, the walls inside of the insulation now act as a
thermal store.
The system I've seen used seemed to be based on something like 50mm
Kingspan, covered with mesh then rendered.