Simon wrote:
What is the easiest way to improve loft insulation, where the loft has been
converted to a room.

Is there a simpler way than removing the plasterboard, inserting rockwool
insulation, and reboarding?

Leaving the plasterboard there, covering it with 50mm kingspan, put new
plasterboard over it.
This is of course both more expensive, and will take up more room.