Do you remember where those rooms are? It's hard to say what they would have been originally without a floor plan to compare to her plans. I do know that the cafe is in part of the space that was originally used for her aircraft hangar. As with most cruisers of the period, Belfast carried floatplanes - in her case, two Supermarine Walrus amphibians. These were mainly used for scouting, but were originally intended to be used for gunnery spotting. As aircraft carriers became more prolific, and the performance of aircraft increased, such aircraft were no longer useful. They were phased out aboard RN cruisers from 1943, leaving Belfast's hangar empty. During her refit in 1944-45, the hangar was converted to accommodation spaces. Her catapult was removed, and the deck space thus gained was used for the ship's boats. Her hangar remained as crew accommodation throughout the remainder of her career with the RN. Converting it back to a hangar space was both too expensive for both the HMS Belfast Trust, and the IWM. Additionally, it would have been somewhat dissonant to make such a change, while retaining other post-war features, like the 40mm Bofors guns, the lattice masts or the enclosed bridge structure. As such, given that accommodation spaces are not the biggest draw for a museum, and that there were no dearth of them aboard, it was felt that it was possible for this space to be turned into a cafe. If the other rooms you mention are in the same section of the superstructure, the history and motivations were probably similar - the hangar took up a great deal of space, and the cafe only fills a small part of it.