Advice on how to correctly insulate a single skin/brick utility room?

Just after a bit of advice off the handy Andys of the stw world :D

I need to insulate our utility room, it’s a single skin/flat roof extension that connects the garage to the kitchen. There’s also a downstairs toilet attached so this will need insulating too, I’m going to add a rad/towel rail in there for some additional heating. Up to now I’ve ripped all the old flooring up and removed all the wall tiles (it was fully tiled for some reason).
Now what’s the best method of insulating, the room(s) aren’t huge so I don’t want to take up too much internal space. I was thinking baton out and 50mm celotex, does a vapour barrier go straight on the brick, then baton out with standard plasterboard on top (no need for the foil backed stuff?)? Any alternatives?

Cheers!

Might be easier to do with insulation backed plasterboard? Reduces cold spots and should save you time as you’re not cutting insulation to fit between studs. It generally comes with a built in vapour barrier. Plus as the insulation is in direct contact with the surface covering it’s more efficient (with the best will in the world you’ll end up with little gaps between the back of the plasterboard and the celotex).

Dot and dab some insulated plasterboard straight onto the brick. No need for any timber. Use foil backed and there’s no need for a vapour barrier. Best option is external insulation though but that’s a bit more complicated.

Taped up Celotex is vapour proof, so you don’t need an additional membrane. No vapour is getting through 0.5mm aluminium.

I’d be careful with dot and dab if it’s only single skin as you might get some.moisture coming through, which won’t do good things to your adhesive in the long term. Also, unless it’s a permanently/regularly heated space then keep the insulation as close to internal coverings as possible.

Who’s the bloke the built the monster man cave? He used insulated plasterboard and then used some self taping plaster board anchors, I was going to do exactly the same in my office, but I’m moving so I’m not now.

We’re doing a garage extension which is single skin wall.
We have used 18mm ply against the brickwork then studded it out.
Building regs have specified 50mm celotex with 32mm insulated plasterboard over that.