Have to say until that photo I have never seen or come across a dry lining box installed before the plasterboard.
PS I like that you have labelled every cable. One thing on that I'd do would be to have the label inside the dry lining box so it's still there for future reference.
PPS Why flex cable for the fan?
In all honesty, whilst I wish I could claim I'd invented an entirely new installation technique on my first attempt. In reality it was because I hadn't a clue how long (short) to trim the cables to fit, so I thought I'd better put everything in situ to avoid problems later. It will all be taken off for boarding over the stud work but at least the cables will have found their shape, so to speak and fortunately it did fit, just, and without too much pressure on the connections.
The run to that backbox was difficult. Being on the 3rd floor (eaves), I had to take it on the underside of the paper backed insulation, which has battens. This is above the studwork top plate. It was fiddly for a first timer like me. Ended up using 25mm conduit as a precaution against studwork screws and/or later work and to avoid the cables moving outside of the permitted zones, especially the flex. All that added to this afternoons drama but I did find out that a PEX pipe cutter does a decent job on conduit.
I will move the labels yes, had them labelled for the pull through but as you say, would be useful to have them in the backbox itself, or just slightly above.
How much slack is it normal to leave? Or is that literally a case of "how long is a piece of cable"?
Flex cable for the fan because it has a round punch out on a rubber grommet as a cable entry point, then a rounded clamp further inside the termination housing (terminology?). It's not perfect. I found flex annoying to work with. All the strands, a real faff for someone as cack handed as me, especially into tiny terminals, but it being 1.5mm didnt help with the terminals clearly being designed for 1mm.
We talked at home tonight and I think we are going to rewire all the flex to the fan and LED drivers as 1mm (assuming proper cable spec), and use some little crimpy things on the ends, to effectively turn the multi strand into solid wire at the termination. If you know the name of those little bits of copper and what I need to crimp them down, I'm all ears!