Ian_m wrote:
In an effort to make ignoring Part P easier (sorry re-wire kitchen) I have
removed the plasterboard ceiling which was 15mm plasterboard.

That's unusual!


Should I just replace with plain 15mm or would moisture proof and/or sound
deadening (as my sons bedroom is directly above) be more suitable ? Any
thoughts ?

You could apply two layers of plasterboard, and get a 25mm thickness.
No point in using anything special.