it is not just the 400k difference, it is the difference in weekly sales. According to GFK the Xbox360 outsold the Wii 2:1, which is insane.
Here is what VGC said:
http://vgchartz.com/hwcomps.php?cons1=Wii®1=UK&cons2=PS3®2=UK&cons3=X360®3=UK&start=39852&end=39985&weekly=1
You can just change Wii and Xbox360 + get the PS3 slightly up.
Imagine not having GamePass on your console...