Mark
I would not skim over that.
That looks like lime plaster and cement plaster patches.
And I bet the lime plaster has blown (just hanging onto the wall - sounds hollow when you tap it).
In my house where a skim was put on that, eventually the differential movement of the lime plaster with the cement skim means that new cracks form.
Also when I drill into it the (for say a picture) the skim peels away around the drill hole and the loose lime plaster makes a massive hole.
Take it back to brick (use a SDS drill with chisel) and re-plaster with insulated plaster on external wall (what I normally do) or board over the lot.
Our house is also 1902. Big refurb. All external walls taken back to brick as all internal plaster was hollow. Boarded in insulated plasterboard. All good - house is toasty.
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