I think that both Daz and I would agree that the build up on your car has taken place over many years and is quite heavy. I would expect the roof to take about two hours just to bar. So it could take allday (sorry mate)
You must wash the paint work first, and then foamwash if you can and wash with clean warm water. After barring you have to wash the paintwork again.
Once this is done the paint will be stripped of all polish/tar/grim and will need sealing with say Sonus Gloss Enhancing Pre-wax Polish which will help to remove fine swirl marks. Then finish off with Victorian Wax applied with your fingers or balm, not a cloth. Allow to dry and buff with a microfiber detailing cloth.
It might be a case of start with a small area first, say the boot lid, just under the spoiler.
YOU MUST BE VERY CAREFUL as the clay bar is an abrasive and you must have enough oil on the paint and bar to allow the clay bar to slide over the paint.
Do a trial area, say low down on the bumper.
What we need is a workshop day.