"We have the typical 'Green' coloured P5 flooring throughout the house."
This is chipboard and is NOT a suitable substrate for tiling onto if it it is remotely possible it will get wet.
"Would it be 'Good Practice' to lay 6mm Plywood for 11mm thick tiles plus an adhesive thickness of 5mm."
No - 6mm is no way thick enough over chipboard. The important thing about tiling an upstairs floor is to minimise movement. You won't completely eradicate it but ALL wooden floors move and if you don't take precautions the grout and probably the tiles will crack. If you must overply the chipboard use minimum 18mm. Yes you'll get a step at the room transition but thats better than a ruined floor.
IMHO You should rip out the chipboard and replace with 24mm WBP Ply. Screw it into the joists at 150mm centres and ensure that additional noggins are put in where the joins are not over a joist. It is not necessary to go to the expense of marine play. Do NOT use OSB!
I would also use a decoupling membrane like Schluter Ditra over the ply to allow lateral movement of the floor under the tiles. Fix it down with flexible bagged adhesive and then tile over that again using the same flexible adhesive. Do not use tubbed stuff. Use a flexible grout as well.
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