Historical version 1 of Coordinate System (view current version)
Geographical coordinate system are hard.
OS X and OS Y coordinates have a number of translations into latitude and longitude, using different 'ellipsoids'.
The convert tool [http://london.openguides.org/convert-coords.cgi|convert-coords.cgi] in the past used the "Airy" projection, and is now using the WGS-84.
Most GPS systems use WGS-84.
Most online mapping systems use WGS-84.
  This is version 1 
  (as of 2005-11-28 20:39:50). 
  View current version.