6mm plasterboard is not very common - try richard russell in croydon on the beddington park trading estate; tel 020 8684 7575
they specialise in sheet building materials though mostly timber products. they supply most building and timber merchants of their sheet materials and are the largest sheet supplier in london.
i know they do a gyproc 6mm flame retardant/moisture resistant board, not sure about ordinary plasterboard. i usually get my ply and mdf there as they do bigger sheet sizes than the standard 2440x1200mm.
if they don't sell it then i doubt that anyone will but i'm sure they will be able to tell you who will stock it if they don't.