from here
Coordinate converter | British Geological Survey (BGS)
Convert British National Grid (BNG) formerly known as the National Grid Reference (NGR) to latitude and longitude (lat/long WGS84) or vice versa.
See The National Grid FAQs for an explanation of how the BNG works.
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Longitude: -0.127216941073 using 530072
Latitude: 51.506418648 using 180272
or
Longitude: -5.40474958046 using 180272
Latitude: 54.6178411829 using 530072