This feature is available only the higher-end hiking-oriented watches, like the fenix series, and on the 935, since it's practically a fenix 5 in a plastic case. AFAIK, no other garmin watch has this feature. You might have some luck with an IQ app, though.
If it could do everything a 935 could do, it would likely cost as much. In Hike2, I allow 4 formats for lat/lon, but no country specific formats (well, except MGRS which is native to the watch.) There's no ABC widget, but there are CIQ widgets that do that. And there are DFs that allow different formats for lat/lon. And for sensors, there is a difference, in that the 645 doesn't support the batch mode HR possible with the HRM-Tri and HRM-swim on the 935.
My point is that the software on the 645 seems to have been deliberately limited, presumably to move people towards the more expensive Fenix series. It's frustrating for people like me who want a do-it-all watch but want it to be light for running.
Of course it's deliberate and it makes good business sense. Why would Garmin make Vivoactive, Forerunner and Fenix series devices that all have the same capabilities? Device size and weight all depend on the watches capabilities and hardware needed. What you feel would be the perfect watch wouldn't be the same for everyone. As it stands, you have to make a choice on what features are most important. Do you prefer a lighter watch for running, or are the bells and whistles more important to you?
On the 645 I can't seem to find any way to change the position format from the default WGS-84 to British National Grid. In the 935 and Fenix there was an option to change this under System > Units. Now, when I look at my Saved Locations of places in the UK, the locations are all in WGS-84 degrees.
Hey Scott. I'm not familiar with the British National Grid. Would this widget help
https://apps.garmin.com/en-US/apps/07b03259-24c8-4828-b949-2e48ef04a2c5#0