You can use Grid InQuest II to transform a single coordinate or a batch of coordinates. It incorporates a wizard so you can even define your own input file format.
Grid InQuest II is available to install directly onto a PC, which means it doesn't have to be used online. The software is compatible with Microsoft Windows (XP, Vista, 7 and 10) 32 bit and 64 bit, OSX and Linux 32 bit and 64 bit. It is licensed as per the BSD Licence.
OSTN15/OSGM15 replaced previous models OSTN02/OSGM02 in August 2016. Read about the updated OSGM15 and OSTN15 transformations for UK and Ireland (PDF).