You won't achieve 0.18U with 50mm of insulation, unless you used a Vacuum Insulation Panel (which are still very new, very expensive, and rather impractical for roof situations).
PIR is the most common material, with a lambda of around 0.021 at the very best, 100mm of this would give 0.21U. If you can afford the extra, Phenolic foam will give a slight reduction on the U value, but only slight.
Consider 50mm within the rafters and 50mm below, which would of course give around the same U value, but since the rafters would be covered, you will have reduced the effect of thermal bridging of the rafters themselves, which would give a better all round insulating effect.
Your final viable option, which again is more expensive is Aerogel (
http://www.thermablok.co.uk/products/th ... el-blanket), which has a lambda of 0.014, meaning 100mm will give 0.14U.
What ever you do, don't use the multifoil insulation, if you read closely their stated U values can only be achieved by using PIR insulation as well as the multifoil. Its just a gimmick!
Hope this helps!