Wiring should be done first, otherwise you will have to lift it all up again.
Wiring should not be buried in insulation. It can be under the insulation, against the ceiling; or it can be on top of the insulation, exposed to the air. In a loft most of it can be fixed, exposed, to the loft timber above the floor. It does not have to be hidden.
Lighting circuits are mostly very lightly loaded, with energy saving and LED lamps, but an immersion heater circuit, and especially the cable for an electric shower, will be carrying full load for extended periods so must be able to lose heat.
In a loft, I think you will need a few lighting points, both sides of the loft, middle and both ends ds; and i think you will need sufficient socket outlets to plug in power tools and a vacuum cleaner without needing extension leads.
It's possible to have a master switch with an indicator neon, fitted outside the loft, next to the hatch, on the ceiling, so you can see at a glance if the power has been left on.
You need a qualified electrician who is a member of a self-certification scheme, and preferably of "contractor" grade rather than just "domestic installer" which is a lower grade.