I found this info online about Penny Mordaunt's dress:
She told POLITICO in an interview that she “felt it wasn’t right” to wear the customary black-and-gold court dress of the privy council sported by the Marquess of Salisbury when he did the job for Queen Elizabeth II in 1953, and wanted “to come up with something that is modern and will give a firm nod to the heritage” of the occasion.
Mordaunt has commissioned a dress stitched with the fern motif of the privy council which has been adapted and “feminized” by specialist atelier Hand and Lock, whose embroiderers have been working overtime changing all the cyphers on uniforms ahead of Saturday.