High performance insulation

[lalalonglegs]

I want to insulate the ceilings in a bedroom built into an attic. If I use the full 270mm recommended by building regs then it will really impede on the ceiling height - are there any thinner types of insulation that have good U-values (I can never remember if U-values should be high or low to be desirable )?

[PigletJohn]

Celotex and kingspan are better insulators than mineral wool quilt. You only need something like half the thickness. They are rigid boards. Have a look at their websites.