We have added British National Grid support to the online map at what3words.com
Click the profile icon in the top corner of the screen and select ‘Share Settings’ and then ‘British National Grid'. Now whenever you click ‘Share’ in the side panel, you’ll see the location’s grid reference in the preview box.
This functionality isn’t currently available in the what3words app. If you have a specific use case in mind, please do get in touch to tell us more.