My house more than likely predates BS anything, but I am well aware that ceiling joists have a much smaller design load to support than floor joists. Professional advice is probably necessary, rather than all youse internet experts! :P
When was your house built? Some were built stronger than others. It depends what you plan on putting up there, and if you want to be able to walk/crawl around too. Get a structural engineer round if you want peace of mind.
according to BS6399-1:1996 Loading for buildings
loadings to be:
Roof
Dead Load 0.85 kN/m²
Imposed Load 0.85 kN/m²
Ceiling
Dead Load 0.25 kN/m²
Imposed Load 0.25 kN/m²
Floor
Dead Load 0.50 kN/m²
Imposed Load 1.50 kN/m²
Walls
Partitions 0.50 kN/m²
Blockwork (100) 1.80 kN/m²
Brickwork (112.5) 2.50 kN/m²
example here