If the original roof space inspection tells a max head height of below 2.3m, there are two methods to resolve the situation, but both solutions will require the involvement of experts.
You could eliminate part or whole of the roof and reconstruct it to the necessary height and structure, but this is expensive and needs obtaining the necessary planning permission. You will also be required to safeguard your house from the adverse weather during the renovation using a sheltered scaffold structure.
Otherwise, you can add height by dropping the ceiling of the area underneath the room, if you can preserve the height 2.4m or more. Getting rid of the existing ceilings can be a challenging project, and a steel plate will need to be fixed firmly to the wall for the new floor trusses to suspend. You will also need to fix up a fastener that ties the new ceiling to the roof to avoid the roof from spreading.