Garden room brick/block advice please

[Marlboro2026]

Greetings :
I am replacing an old asbestos garage with a garden room. The footprint is sizeable 30m2, within permitted development and contacted my local planning regs office already. 

I decided I would hire each trade individually, I have a limited budget for this project. 

My dad was a sparky and I lost him last year. Whilst a sparky, he would turn his hand to lots of things at home and was my go to person for everything, I am a pen pusher and miss him. I am hoping for some guidance as to what stages I should be doing things and some recommendations on the build.
It will be used for storage and a home office. Electricity, no plumbing.

I hired a digger who has demolished what was there and he will move on to the next stages :
Footings 450mm x 450 deep & wide
Solid floor / type 1 hardcore + 150mm concrete with mesh + 50mm screed

The next step is block & brick, I think some form of insulation between the two?
If the height is 2.5m, width 7m and depth 6m, how many engineered bricks do I need, normal bricks and blocks?
The guy who is doing my foundations initially told me he has a couple of bricky friends and it would cost £80m2, it then became £120m2 and would take 2
months to build because brickies can only lay a certain number of courses for stability or it would collapse. I Googled this and accept it is now winter, but 2 metres at a time seemed acceptable? Since the height is only 2.5 metres, I am confused.

Do I engage a roofer at the same time as a bricklayer or once the walls are complete or nearly complete, I am not sure if technically, they somehow work in unison?
The door/window fitter, is this something the bricklayer would do?

I am going for a flat roof, what sort of wooden beams do I need?
EPDM, GRP or felt roofing and what is involved and what sort of materials should I be looking at sourcing?

Attached is an AI generated photo of what I would like to achieve.

Appreciate some guidance. I am not cheap, but my funds are limited so I am trying to get a good build for a fair price.
Many thanks in advance.

[Onoff]

I'd think about insulation under the floor, like 150mm min. If you're laying a concrete slab then why not incorporate UFH pipes now even if you never use them? 

 

You'll want the walls insulated too. Pointless not too. 

 

At that size I'd have put a trench in with soil & water pipes coming up through the slab as future proofing. Decide on your plan where you might later on want a WC / shower room.