Just a technical note. There is no such thing as a lat/lon without a corresponding geodetic coordinate system.
The crux really is that the phone’s GPS has a native and open CRS (WGS 84) that can be used to convert to and from other global or local CRS such as OS National Grid. An interesting fact is that many parts of the former soviet union have yet to repeal laws making it a criminal offence to convert anything from specified CRS. It’s not even anything to do with “secret” locations, it’s illegal to convert unless you are licensed. These laws are very similar to the TOS for W3W