If you want a speedy reply from a manufacturer contact Xtratherm with your proposed roof construction and ask them to specify the minimum insulation thickness for your target u value. Kingspan website is good for literature with pretty pictures though, and there are tables in there showing how much insulation you’ll need to achieve a certain u value. You can use insulation from any manufacturer as long as the thermal properties are the same, or similar, which most are unless you go for the more expensive stuff (which you’d only do if you had restricted space to build up the required thickness).
There’s a few ways to go about it but I’d probably have most of the insulation on top with insulated plasterboard inside, but it may depend on which roof cladding system you go for.