Two options are open to you. You could fix 38mm x 50mm timber battens at 90 degrees to the underside of the beams using proprietary clips that are hooked over the beams at 400mm or 600mm intervals when laying the floor. The clips are usually available from the floor beam manufacturers if specifically requested. Alternatively, you could ask your plastering and drylining contractor to supply and fix an MF ceiling system to carry the plasterboard as part of his package. This involves boards fixed onto a framework of lightweight metal channels. MF is probably slightly dearer but has the advantage of involving one less trade and a single point responsibility for the work.