Google's UK maps have just gone beta, but it looks like they have a geoid problem.
Typing in a postcode, you get an error similar to the error expected from the wrong geoid. It looks like their postcode database uses grid references, and the translation from these grid references forgets to account for the geoid. Using them as if they are WGS84 and not Airy 1836.
An example is here:
LS17 8LY actually refers to 10 houses, a pub and a garage (gas station) on Wetherby Road (near where it says Wellington Hill). They're too small to show up on the map. Google positions the postcode in the middle of a field. The problem doesn't look to be a centroid problem - eg. including Coal Road by accident, because that would give a position further west.
MapPoint 2002 has a similar problem. It could locate postcodes okay, but uses grid references on WGS84 instead of Airy36. MapPoint 2004 fixed the problem. I've sent a message to Google, so they should fix it soon.