Earth is not a sphere. It is an oblate spheroid. What is that you may ask? Imagine taking a perfectly spherical balloon and squeezing the top and the bottom between your hands slightly. An oblate spheroid is essentially a squashed sphere. Consequently the historic Greenwich Meridian (Used by Britain since 1851 and agreed internationally* in 1884) is different to the International reference zero meridian (ITRF?) provided by your GPS.
The method of early calculation used a vertical line determined by a plumb line from which to observe – assuming this plumb line pointed to the centre of the earth. Obviously, on an oblate spheroid, it would not. Furthermore, any local magnetic disturbance would alter the measuring process. The system obviously had flaws.
The ITRF zero meridian – your GPS – being defined by a plane passing through the Earth’s rotation axis, is 102 metres to the east of the historic prime meridian at Greenwich.
What about those helpful markers sprinkled across the Greenwich Meridian Line as it passes through the country? Are you really standing with a foot in each hemisphere when you straddle the lines they indicate?
Not really. As Shown above; relying on the historic Greenwich line there are errors. For example, further north than Greenwich, here in Lincolnshire that 102metre error has reduced but it’s still around 100metres.
Even taking into account the historic error, many of these markers are out, some significantly. For instance, the one shown here on the right, near to the village of Frampton, appears to be 52m West of the Greenwich Meridian which it is supposed to mark.
If you really want to know where you are in relation to the Meridian you would be better to check the compass on your mobile phone, they usually show your coordinates – even then only to about 5-10 metres accuracy. There are more accurate GPS measuring devices but they are not domestic and are therefore expensive.
*Prior to the establishment of an agreed meridian most maritime countries had their own meridian, usually passing through the country in question, from which navigational measurements were made