This project implements a simple REST server to allow conversion between Ordnance Survey grid references and latitude / longitude.
The easiest way is to start the server using Docker:
docker run -d --rm --name osgrid-server -p 9090:9090 paulcager/osgrid-server
A grid reference may then be converted to a lat / lon:
$ curl http://localhost:9090/v4/gridref/SU37241550
{"osGridRef":"SU37241550","easting":437240,"northing":115500,"lat":50.93774069083967,"lon":-1.4713807843405298}
Or a lat/lon converted to a grid reference:
$ curl http://localhost:9090/v4/latlon/50.9378,-1.4713
{"osGridRef":"SU37241550","easting":437246,"northing":115507,"lat":50.9378,"lon":-1.4713}