Since this thread was started, support for OS Grid Coordinates was added to GridImp. For details, see:
http://www.mapping-tools.com/gridimp/index.shtml
One of the examples (Snowdonia topography) uses OS-gridded data.
Use 4-digit grid references for 1km squares.
Note that GridImp only draws the data. So not true territories, but you can get some good maps.
Richard