I have nothing against the metal roof sheets, but condensation would be my biggest concern with them also.
Because of the shape of the sheets (i.e. not flat), you can't easily push the insulation tight up against it to close off the air gap next to the cold metal (which is where the condensate will form). Especially if using non compressible insulation like PIR because it wont fit to the curves of the sheet.
If you want metal, the kinspan panels are a nice way around that as the PIR is moulded 'inside' the sheets in the factory, thus reducing the problematic air gap to zero.
My current thoughts / preference on roofing options for flat roof sheds, in order of preference:
1.
sedum over EPDM (expensive and needs stronger roof=thicker joists/or metalwork=less head height/or even more cost if sticking to 2.5m)
2.
artificial grass over EPDM (100% waterproof, looks nicer than EPDM arguably, not sure on the grass maintenance?)
3.
EPDM (looks 'OK', lasts for 30 years, 100% waterproof and maintenance free, quickly makes building waterproof)
4.
kingspan panels (look a bit industrial, strong /can do big spans, easy and fast to assemble, 100% waterproof and maintenance free)
5.
felt (doesn't last well, doesn't look good, but gets the job done and is cheap, in short term anyway)
6.
metal sheeting (industrial look, potential condensate problem, not always waterproof, not always maintenance free)
GRP should be in there but I don't know enough about it other than it looks complex, messy, weather dependant, specialist etc
All in my opinion only
Martin