Hi my plan is a 18mm ply with 50 mm polystyrene then tyvek then box profile 0.7mm steel roofing , this will be fixed to 75 mm battens to give an air gap to minimise condensation on underside of steel.
Chris
Another vote for Firestone EPDM on top of 18mm ply. If you're smart you order more than you need and the offcuts can be used for finishing off, edge plates etc. It's a one-piece sheet (whatever size you order) so there aren't any joins. I did an outhouse roof about five years ago and it looks as good as the day it was laid. The green Onduline that was put on the other shed roof about 15 years ago is curling up, fraying and generally looking in need of replacement. I'll bet the EPDM lasts a lot longer than 15 years.