The Palm Pixi isn't coming to the UK yet, though O2 which has the Palm Pre is looking at stocking the phone. It's smaller than the Pre and has just 8GB of memory. It'll run the same webOS as the Pre.
Though the OS remains the same, the Palm Pixi will come with new apps for Yahoo Messenger (do people still use that?) and Facebook. Without the navigation button present on the Pre, the Palm Pixi's apps will be accessible by tapping the screen.
The Palm Pixi is due in the US by Christmas while we're still waiting for the Palm Pre which is also coming on O2 in time for the festive season.