insulating a cavity block wall extension

we have a cavity block wall extension(20 ft x 15 ft) and its bloody freezing,we took down the ceiling and got kingspan in between the joists(its a flat ceiling)it helped but its still cold.

the rest of the house has insulation plasterboard on the external walls and ceiling(midterrace house)and the rest of the house is cosy.

i am looking at doing the same job to the kitchen,but the whole kitchen will have to to taken down...........would external insulation be better.

its only two walls and its pebble dashed,can they install it over that or does that have to be taken down or rendered.the other wall in the kitchen has nothing on it so i can internally be insulated.thanks.