Confusion on Northing and Easting values in UK

Question

From what I understand about UTM Northing and Easting values..

In the Northern Hemisphere, Northing is the number of metres from the equator.

Easting is based on the Central Meridian (CM) of the zone having a value of 500,000 thus anything East of the CM would be 500,000 PLUS the number of metres from the CM and anything West of the CM would be 500,000 MINUS the number of metres from the CM

My confusion arises from a Northing and Easting which is in the UK, for example: Northing 172405 Easting 471267

I know that this location is 51°26′46″N , 000°58′33″W (Not very far West of London)

In this example, clearly London is way more than 172km from the equator..

So the Northing of 172495 must be measured from some other point.

Where is it actually measured from?

Answer 1

If you have coordinates in the UK they are more likely to be in OSGB (EPSG:27700) rather than a UTM zone (since the Greenwich meridian is a join between them).

Answer 2

The coordinates you have are in British National Grid and not UTM. As indicated by Ian.

This can be checked using the Grid Inquest II reprojector:

The British National Grid is curated by the Ordnance Survey and is a Transverse Mercator Projection. This projection has false eastings and northings applied to ensure coordinates are positive. See the OS guidance here: Guide to Coordinate Systems in Great Britain

The 'measurement point' is therefore at BNG 0,0, which is to the South West of the Isles of Scilly: