I agree, for long term data such as scientific work or historical records that might be referred back to in five ten or twenty years hence, it would be crazy to rely on WTW as the location point, it only makes sense in that context to use it for making the initial record before transposing it to a lat/long or in this example an UK OS grid ref. Frankly I find Lat/Long a lot less user friendly than OS grid and while I meet plenty of people who cant use a grid ref, I meet a lot more who can’t use Lat/Long. Having said that on iphone at least I don’t think you even need an app as if you enable location sharing in settings the long/lat of any photo you take is stored in the file information so that lots of programs can read it (e.g. in Windows ‘Photos’. click on the three dots for a drop down options menu and select ‘file information’, if it has been enabled by the photographer the location will appear in numerical format and on a map.