I found these guys very helpful even when I wanted a single 8x4 of veneered mdf. I've misplaced the catalogue so I can't confirm if they do v-ply but they do most things. I found them the cheapest locally.
Panel Supplies (UK) Ltd.
Harvey Road - Basildon - Essex - SS13 1ES
Tel - (44) 1 268 729100 Fax - (44) 1 268 729700