I put 50+30mm in total on ours, and the BCO passed it yesterday ;-))

A quick check of the tables shows you're right. The 50+30mm of insulation is
absolutely right on the nail for 0.3. 75mm wouldn't have done it.

The problem come when you install between rafters only. They publish figures
for 75mm and 100mm, with the 75mm being well out at 0.36, but the 100mm
being well in at 0.28. My intended roof (75mm between and 25mm underneath)
gives 0.25.

Christian.