They should also do a tightness test at gas meter before and after installation. This proves no leaks before then after. Ive had so many joints crack while unplugging or plug in the flex, also had a new flex that was split.
Then because the gas has been turned off relight and visual check on all gas appliances in the house.
I'm pretty sure it's illegal to fit a gas cooker yourself, unless you are gas safe registered of course. It is a simple 2 minute job to do it yourself I agree, but what if something isn't quite right, or it's been damaged in transit maybe. I've fitted loads when I was registered, and know exactly what I'm doing, but it wouldn't be worth it as the penalties could be ginormous if you're unlucky and something goes wrong. I'm not sure if it's still the same but, when you finish a soundness test, you label the meter with your name and date on it.
Reading instructions on internet you can disconnect the cooker and block it off with a bayonet connector but you are not allowed to connect the hose to a new cooker at cooker end theres lots of stuff on the net about it.
thread on the end of a flexi hose to the cooker is tapered and if you use a bit of gas safe tape as well its pretty hard to get a leak, if its a new hose, the push fit bayonet plug should be safe enough anyway due to new seals
always use a bit of dilute washing up liquid to check for leaks when installing
you cannot fit a direct solid piped connection to stand alone cookers thats a big no no
solid piped connections are for built in fixed units and you must fit a gas cock somewhere easily accesable to use those
most gas fitters will do a drop test before any modifications and fitting and then one on completion, the first one is to check if theres any gas leaks before works started