Options for dry lining stairway wall

I am currently looking into dry lining all internal external walls of our 1950s semi-detached house and availing of the SEI grant. One of the external walls is the staircase and I am concerned about losing space on the stairs. The current width of the stairway is 800mm. I am worried about making the stairway too narrow and don’t want to damage the character of the house.

As I see it the following options exist:

1. Internal dry lining on stairway all with narrower width of Kingspan to avoid losing too much space on the stairs.
2. Remove some plasterwork on stairway to make more space and then insulate over this.
3. Insulate all other external walls internally, but insulate staircase wall (gable end of house) externally. Does SEI accept combination of internal and external insulation?
4. Insulate entire house externally.

What have others done on staircase walls? Do you have any recommendations on how best to deal with this?

Thanks in advance.