Flat roof refurb

[Creakyoldhouse]

Hi, so my warm deck flat roofs need to be completely rebuilt down to joist level (no firrings, poor build up not in accordance with regs) Will this work destroy my ceilings below? Should I completely remove fittings and kitchen from underneath or can I just protect them and work really carefully. There is an option to keep plywood bottom deck and use tapered insulation but we have limited depth to work with so hoping to use super thin insulation like Optim-R to minimise height. Any experiences welcome. Thanks

[Mr Punter]

I think the Optim-R is ferociously expensive.  What is limiting the available height?

[Creakyoldhouse]

Haven’t priced it yet so thanks for the heads up. 
Parapet wall height and window sill are inadequate to allow tr27 to get to the new regs u value. Need max 120 insulation. 

[scottishjohn]

pictures?