Few Quick Pointers;
Colours - Never use black as a Neutral, it's always grey or blue. It used to be black but they changed it and it gets messy on older installs. Always mark up the core with what it's intended for. (Green yellow for earth, Blue for Neutral, Brown/Black/Grey for phases)
Cables - Have the terminations in the blocks follow the order of the breakers. E.G 2 x Cores in the 1st breaker, 2 x cores in the far right terminals of the earths and neutrals. Good Practice.
Terminations - 1mm out is fine. on single terminated cables its good practice to strip a bit more back and fold the copper over to make a hook shape to ensure good connection at the terminations. You're putting in a circle and clamping with a square, doubling the surface area ensures a better connection. Also when trying to hide the length in cable it's better to loop it down in to the consumer unit and over mid-way in to the breaker. If you stuff everything in to the top/bottom/side you leave no one any room in the future to drill in to the unit without stripping everything back out.
The Armour Cable - Wrong type of gland for terminating it in the the Consumer Unit. The steel strands serve a purpose and should be terminated properly and connected to the Earth Bar via a fly lead or pig tail.
If you're interested in being a spark then buy insulated hand tools and decent brands, they'll make your life easier. The basics are easy to master, if you wanted to prove the theory you can buy a cheap continuity meter and have a crack at the testing to understand the principles. Plenty of material online.
It really doesn't matter what colour you use for wiring aslong as you mark it.
And three phase is the colour code of this cable, i.e. brown black grey.
In industry where we use this cable a lot, we normally wire Brown- Brown Black- marked as blue for neutral Grey --- CPC
1mm out would be a massive fail on ex rated but in this its OK, as long as its in a clean dry area and the box is above ip3x. I tend to like not to see any copper, but i also bootlace crimp all my connections so im just a sad fart
If hes not entering the box regularly then the length of the tails is fine, will be bugger to test but will make putting the cover on easier.
Armoured cable would be better in a proper Gland. If you want to buy one, get a nice 3 part gland, they are a tad more expensive but a lot better in design and fitment. You should have a banjo (brass disc with a hole and tale) with a earth lead to the earth bar.
If you go down the swa gland route, a quick tip i can give you is where you want to trim the swa back to wrap a peice of 20mm electrical tape. The top part of the tap is where you score the cable with the junior hacksaw and the bottom is where you strip the outer sheave for the gland.
Also it gives you a nice straight line to follow so you get a good connection for the gland to grip.
I think the most important thing he should invest in is a voltage tester and test unit, you "dont" need insulated tools if you can correctly isolate and lock off.
Once hes got proof test proof down and lock off then he will atleast be safe to work on other circuits.
Probably would be best to pick up a onsite guild.
But overall, ive seen apprentices do a lot worse than the last image.
oh and id wrap a bit of insulated tape around the spare way prong, just incase the front way blocker falls out or is prised off by a small person
. And definitely as you've already said, get someone to validate your work and power this up. Also if you're in the UK you actually NEED a sparky to sign this off, as it's covered under Part P of building regulations due to it being an outbuilding (assuming this isn't a fuseboard swap...)
