Have you thought making a pig's ear out of this problem? ;)

If you feel a handrail is necessary (I feel it may be difficult to not come to that conclusion), a pig's ear handrail may be useful This one protrudes less than 70mm and will have a minimal impact on the overall width of the staircase as it would be a hip height and in any case, flow down this staircase will be in single file and the handrail will not impact that  And at £15 per metre it wont break the bank

https://stairpartsdirect.co.uk/range/pigs-ear-handrail-price-per-metre/?attribute_pa_timber=Pine&attribute_pa_sd_length=2100mm&attribute_pa_infill-width=infill-width&gad_source=1&gclid=CjwKCAjwkJm0BhBxEiwAwT1AXGjnxyWeykdYAe_q-La_7_3TXGwroGTT8xMbfOHUuQ27rkgBabBufxoCn9kQAvD_BwE

Hi Mosh

This is probably a conversion, and an issue (including the width of the staircases) which should have been considered at the time of conversion, but perhaps you are where you are. 

Have you considered talking to Fire and Rescue and then to Building Control?

Perhaps there is a solution that would enable handrails but still result in less intrusion into what are already very narrow staircases. 

Research by the Health and Safety Laboratory (which is an arm of the Health and Safety Executive) conclude that the value of a handrail is more as something to grab on to if falling than as something to hold on to when climbing up or down.

......and on that basis, I think that perhaps a narrow bar (say 20mm diameter) with sufficient gap between bar and wall might be an alternative to something else.

Imagine a curtain pole but on its side!