Insulating external utility room

[sipit1]

Hi
Have looked through lots of posts, but can't seem to find a consistent answer. We have just moved and have a lovely 1980s breeze block garage attached to the side of the 1850s solid brick house. The back part of the garage has been partitioned off with the idea of turning it into a utility room. It is er approx 4m x 2m across and has a max roof height of 2.5m. It is double layer of breeze block with insulation in between.
I would like to add some kitchen units, sink etc and make the finish slightly nicer looking than the breeze blocks. It seems freezing cold out there at the minute so I am thinking I should add some insulation and then plasterboard the walls.
Can I screw 50x100 timber battens to the wall, add 50mm celotex in between and then plasterboard screwed to the battens? Kitchen units will be ground level and no cupboards attached high up. The ceiling is bare rafters and I would like to add some insulation in between those as well. Thought I could use the same technique. Insulation between the rafters and then plasterboard screwed to the rafters.

Am I over complicating things, is there an easier solution?
Would I need to inform building regs that I am internally plastering some walls?

Thanks for any help.
cheers
simon

 

[noseall]

Use 300mm insulation at ceiling level i.e. 100mm between rafters and 200mm across in opposite direction.

Use 50mm insulation across the battens not between, for the walls.