I'll flesh this out when I've written a script to convert TM coordinates to BNG, but for now use the following proj command to convert from lat/long to TM

proj +proj=tmerc +ellps=airy +lat_0=49 +lon_0=-2 +x_0=400000 +y_0=-100000 +k=0.9996012717

Then once that's running you enter the lat/long to be converted at the prompt in x y format (ie long then lat) and it will return transverse mercator co-ordinates in the BNG co-ordinate system (still have to convert to the right OS grid square though).