I think you need to look vary carefully at any treated paper option.
Carefully examine any agrement cert, guarantee or assurance and vet the company thoroughly and its portfolio of work.
I saw a house nearly burn down once because a recessed downlighter overheated and the treated paper insulation caught fire.
What stands up to a high flame in open air where hear can get away may behave differently in a confined space where there is already a heat build up.
Be very carful about maintaining the required 50mm ventilation air gap from the eaves and a good read of Part F is recommended before installing.
This is particularly so where any rooms are "in" the attic/ follow the roof and remember that quited insulation can sag and bunch.
Compressing insulation is never a good idea - this increases the solid material present and reduces the amount of traped air.
Since its the air that provides the insulation, you are shooting yourseld in the foot by flooring out if it compresses it.
If your going the conventional above the ceiling route, installing a vapour check is a good idea.
Do not leave uninsulated pipes exposed above the insulation.
Do not cover over downlighters or their transformers.
Do not create an unsafe place to work in later on.
DO NOT INSULATE UNDER THE WATER TANK.
Insulate around the tank and cover it.
The cover should be insulated.
Of course, you could go a slightly different route and insulated between the joists and beneath the joists, assuming you have a "clear" attic.
This will keep the insulation layer to the outslde of the building - the envelope - and pave the way for a later full conversion.
You should consider the likely floor loading, whether for store or habitable room and upgrade the joist depth to suit.
HTH
ONQ.