after 30 years of building and plastering my own prefered choice.. (I am also a qualified BER assessor) would be medium density block with a 125mm cavity fully filled with Polypearl platinum bead, internal wall of 100mm Quinn lite block drylined with 12.5mm tapered edge plasterboard using dryline adhesive.
This method allows for the best of all worlds.
You have quick response heating time as you are not as mentioned heating cold internal poor u value blocks.
You have longer term, (when the heating is on for longer periods), thermal mass which in my opinion can not be overstated in importance, virtue of the excellent u value aereated concrete block (Quinnlite)
You have full fill top notch insulation at 125mm.
Cost considerations this method gives the best value for money and the best benefits, after all these years I like to think I know a bit about it all....... Not much else mind !!