It sounds like you are as well to go for Building Reg appro, you could go to all the work of putting in windows, only to get told to take them out again
I am following BR in my current project, I nailed 1"sq batons 1" down on the rafters and am pushing about 2" insulation between the rafters up to the batton, hence the required gap, so I only need a further 2" screwed below to meet requirements. I was lucky and got some 2" insulation with plasterboard bonded from Seconds and Co for a very good price, so I just screw this lot up, get it skimmed and that's it.
I found the Building Control Dept in Bradford very helpful, I took lots of photos during each stage of the insulation process, and when I went to see the inspector for the half way thro checkup, I showed him the photos, and he was happy enough with what he saw to say "just proceed, no need for a site visit"

Also when you come to sell the house, you can show the photos to any HIPs person to prove it meets BR standards

Ken