wifi only ipad will never really know its accurate location. It has no GPS.
What it does is look a what wifi hotspots it can see, then it check online if any of those hotspots have a registered location with skyhook.
The wifi hotspot locations dont update automaticaly so if any of the hotspots move the ipad will think they are in the hotspots old location.
It could very will be that your hotspot is registerd in lonon, or one of your neighbors has a hotspot that was registered in london and then moved.
Your iphone gets good location because it actualy has a GPS built in (3g,3gs,iphone4)