Edit - Update with OSGB36 specific information.
Well, the good news is that the target projection is OSGB36, which is a well known and standardized projection. The bad news is that GIS programming is still really difficult :)
There are a set of parameters that need to be defined, and mathematical operations applied to those parameters, to succesfully project lat/long to grids. Fortunately, there are databases of parameters for the former, and libraries for the latter. proj.4, which I mentioned earlier, is one of the standard libraries. There are also ports of proj.4 to other languages, including JavaScript, which is hopefully close enough to jScript for your purposes. The port is proj4js.
The other thing you need to know is the EPSG number for your coordinate systems. OSGB36 is EPSG:6277. The EPSG is an oil-industry (I think) created body that collected lots of locally defined projections and documented them. The database is now openly maintained, and GIS libraries can use their definitions.
Previous info
Your easting/northing numbers may be in the British national grid system.
I gave an answer to another GIS related question that includes links to proj.4, which is one of the standard libraries for converting between lat/long and easting/northing (the forward calculation is called projection in the GIS world). I've never used it for the conversion you're looking for, but the software is well-documented and comes with conversion definitions for lots of projections.
As you're dealing with a very large curved region, you cannot get an array that has regular intervals in both angular units (degrees Lon-lat) and linear units (meters E-N).
The topic of map projections concerns this mapping of the curved earth to a flat map, and hence geographic coordinates (lon-lat) to map coordinates (E-N).
The topic of spherical geometry concerns conversion between geographic coordinates and linear distances.
This question may help: calculating distance between two latitude-longitude points.
Things to consider:
Do you want fixed linear intervals (not really compatible with what you're starting out with), or a fixed number of intervals and hence fixed angular intervals?
Depending on that, you may need to calculate the three side lengths of the "rectangular" box? The N-S lengths of the left and right sides are the same, while the E-W lengths of the top and bottom are different.
Do you have any restrictions on software you can use?
Best Answer
You can perform that transformation using cs2cs, a tool that is part of the Proj4 package. In case of need, you can find the proj strings for the Indian system(s) at SpatialReference.org.