We have a three storey commercial (office) building. It has an internal protected fire escape with the offices on either side. To one side of the building it has an external fire escape. The external one is very old and does not comply with current standards. This would be in the six figures to be redesigned, manufactured and installed. Can the external escape be take out of service and use the one internal escape. Obviously the signage and strategy of the building will need to be changed.

That's one for your Fire Risk Assessor to deal with. The FRA would look at occupancy numbers, layout, travel distances, other fire precautions etc and determine if the building can comply & remain safe without it. I've assessed various buildings with this situation and some have been able to do without the stair but others have had to replace it.

Many standards are not retrospective and age alone doesn't mean it's unsuitable, why exactly have you been told to replace it - most often it's due to lack of maintenance and becoming structurally unsound being the reason to replace - again your fire risk assessment should take a pragmatic approach as to whether an older style stair is suitable.