So, i believe it could be possible to have a premises that complies with B.regs but is considered not to comply with the FSO.
Nothing new there.
In fact i would even go so far as to suggest that you could meet the design criteria within AD(B) and still fail to acheive compliance with the functional requirement.
ADB gives only guidance. It states, itself, that it is only appropriate for less complex buildings and it points to alternative sources of guidance. It does state clearly what the functional requirements are and, before it goes into specific recommendations, it supports the functional requirements with broader general comments about how they might be met.
Often fire engineers will view ADB recommendations as too onerous and will try to chip away at these but sometimes it is the other way round, as you are referring to, where ADB falls short of providing suitably complex recommendations for a given building. ADB is far from being the be-all-and-end-all.
Regarding the conversion of lifts to evacuation lifts, this is not a simple matter, as kurnal has already pointed out. There are requirements for access and egress routes, including suitable lobbies, that might be difficult to achieve.
I have seen situations where people think that if they put an evacuation lift in they can fill the building with people in wheelchairs - when, in fact, little would have been more dangerous.
Stu