All-numeric OS grid references. 

Numeric grid referencing omits the reference letter prefixes, eg NC,NG, NS, and replaces them with a numeric prefix: one extra number added to both the Easting and the Northing.

The zero reference for this prefix number is somewhere SW of the Scilly Isles and the increment is one per 100k.

So- 

NC=2xxxxx 9xxxxx

ND=3xxxxx 9xxxxx

NG=1xxxxx 8xxxxx

NH=2xxxxx 8xxxxx

NJ=3xxxxx 8xxxxx

NK=4xxxxx 8xxxxx

NM=1xxxxx 7xxxxx

NN=2xxxxx 7xxxxx

NO=3xxxxx 7xxxxx

NR=1xxxxx 6xxxxx

NS=2xxxxx 6xxxxx

NT=3xxxxx 6xxxxx