I can't comment about the Kemble vs Yamaha grands, but I can confirm that there's a difference in sound and build quality for the uprights. When I bought my Kemble Empire I also played the corresponding Yamaha (which was less expensive even though the showroom was dropping Kemble and consequently had slashed their prices on the Kemble pianos). Anyway, the pianos were in the same showroom and were no more than 10 ft away from each other. To my ears the Yamaha sounded thin and shrill compared to the Kemble. My husband, who's an engineer, also pointed out that there was less bracing on the Yamaha soundboard and that it was generally not finished to as high a standard as the Kemble.
My tuner, who looks after many fine grands, always remarks on what an amazing sound my piano has for a small upright, especially the bass, which she describes as 'buttery'. And I'm very pleased with my piano, I've had it for two years, it has very stable tuning and I love the way it sounds. So from my own experience I'd recommend Kemble, with a couple of caveats: their pianos are built in England, so import costs ramp up the price, and there aren't many people who've heard of Kemble over here so in terms of resale you could lose value because of the unfamiliar name.
Having said that, I'd add the usual advice: buy the piano that sounds best to you and feels best under your fingers (and fits your budget, of course).
EDIT: Forgot to say that Kemble is a part of Yamaha Europe and builds Yamahas for the European market. You can check them out on their web site: Kemble pianos