You won't get 30 minutes fire resistance (integrity) from a single layer of 12.5mm thick standard plasterboard on a ceiling. 15-20 mins at most. You can get 'modified' 30 minutes F/R from a timber joisted floor with 21mm softwood floorboards (see the Gyproc 'White Book' - http://www.british-gypsum.com/pdf/WB09_WHITE_BOOK_08.pdf
), but bear in mind that the 30 minutes applies to Loadbearing Capacity only - not integrity or insulation, so it's not providing a fire separating function for the full 30 mins. 'Modified' fire resistance only applies to dwelling houses, though.
One layer of 12.5mm Gyproc with appropriately sized joists and
min 21mm tongue & groove softwood floorboards will give you 30 minutes F/R. If you need 30 minutes from the plasterboard alone, you'll need at least two layers of plasterboard, or use a plasterboard with enhanced fire resistance (e.g. Fireline), with appropriate insulation above. You'd also need to put noggins between the joists to support the board edges.