I would've thought 17k is a decent number considering the pop. of the UK is 60million. Sony should be worried if it quickly drops below 10k. You have to remember that a lot of PS2 owners are casual gamers and will wait for a price drop and for their favourite titles (PES and the like) before they consider upgrading.
As for it being the Wii's 3rd biggest week. Wow! They ship more consoles and Wiis are still going for hugely inflated prices on ebay. Crazy demand!