The posting by seer provided an excellent background on the Prime Meridian. There have been several primary meridians. The attached placemark shows where in England they can be seen.

Bradleys' (1824) is still the basis for the Ordnance Survey maps. It can be seen at the Chingford Pillar, also called Bradley's Obelisk.

Airy's (1851) meridian is 19 feet to the east of Bradley's. That is the meridian tourists see at the Royal Observatory's Flamstead House in Greenwich. It is called the Prime Meridian, Airy's Meridian, or the Greenwich Meridian.

Google Earth correctly uses the International Reference Meridian (IRM) based on the WGS84 scheme. It is a further 102.4779 meters eastwards.

However, Google received some unfortunate publicity because they called that meridian the Prime Meridian when Latitude-Longitude is turned on (Control-L). It should be displayed as the International Reference Meridian.

Another good source of information is History of the Prime Meridian