With out knowing the size direction and spacing of any reinforcement and the thickness of the reinforced slab I dont see how your engineer can know that the wall was not there to reduce the span of the reinforced slab, he's therefor only guessing. Council houses are natorious for there poor...
No not true, does not matter the classification of the room only that the means of escape in case of fire is adequate. Cooking facilites should be remote from access to the escape route unless enclosed within fire resistant construction. See approved document B part 2 dig 2-5 for the internal...
This Government determination may be of interest in that you would need an application under building regualtions if your using the space for anything other than maintainence of the roof.
Requirement B1: Determination of compliance in respect of alterations to the roof space of a two storey...