Large 12-inch tablets are not quite as rare as hen's teeth -- but almost. If you want a 12-incher running Android there are two Samsung options, the Galaxy TabPRO 12.2 and Galaxy NotePRO 12.2, both with 12.2-inch screens, while Windows fans can opt for the 12-inch Surface Pro 3. For the record, HP's Pro Slate 12 (£418 ex. VAT, £502 inc. VAT) beats all of these on screen size with its 12.3-inch display.
HP Pro Slate 12 review at ZDNet
HP Pro Slate 12 review at ZDNet