100mm of Celotex is approx equivalent to 225mm of itchy stuff...... 2 and a bit times the value.
Floating floors are used all the time Slackers. No need to fix down.
Usually the T&Gs are glued. A Fairy Liquid bottle full of PVA is ideal for squirting it into the grooves. That's in a normal domestic situation. In a roofspace, traffic is so light you could happily leave out the gluing.
It's an excellent idea, and you should certainly feel the benefit.
ps.... anywhere where it's just too awkward to cut and position Celotex, don't forget to stuff with plenty of ordinary loft roll.
Alternatively, I could point you to a special type of itchy roll that has a much higher "R" value (resistance to heat loss), but it is rather silly money I'm afraid.