Single brick wall insulation options

Hello, perhaps you can help - first time post...

I've got a house built in the 70's. The style of house means the lounge and bedroom window (above the lounge window) have been built into a single thickness block wall with tiles on the outside between the two windows. The rest of the house is built with a cavity wall.

I'm looking at insulating the bedroom and lounge wall from the inside. Just trying to understand what my options are and the best way to do it. I don't mind losing 100mm of space if it means better insulation. I'm aiming to put a curtain rail above each window and there will also be a radiator below each window, so this is something that will have to be factored in to support them.

I've attached pictures for reference. As you can see - the window boards/sills are quite narrow on the inside, so understand these would have to be widened. Also looking at getting new double glazing, so do I change windows and then insulate/decorate up to the new windows, or do the insulating/internal bits first and then get new windows?

Any help, much appreciated.

Anyone got any ideas?

Has to be Celotex/Kingspan then plasterboard doesn't it?

cover the whole wall with 50mm celetex type insulation, boards tight together no gaps, foil tape all joints.

screw on 50 x 25mm battens laid flat (maybe 75 x 25 for radiator brackets). Note that plasterboard is 1200mm so set out your battens to suit

12mm plasterboard and skim.

You will need new windowboards.

Do wall before new windows, but dont skim plasterboard.

Fit windows, then fit windowboards.

Then plasterboard reveals

Then skim.

You could even fit 25mm celetex to window reveals to minimise thermal bridging, provided the new windows have big enough frames -you dont want to end covering the frame moulding (best to remove reveal plaster before to avoid losing too much space).

Thanks for the response.

All makes sense.

Celotex just stuck to the wall with suitable 'no more nails'?

Also, with screwing the battens to the celotex - does the screw just go into the celotex, or all the way through it and into the wall behind?

Apologies for all the noob questions.

Screw right through it into the wall, the battens will hold the celotex.