In reply to TL2309:
For the newer Garmin watches where you can add apps/widgets/data-fields via the Connect IQ store then there is a Data Field called OS Grid Ref that you can add to the watch https://apps.garmin.com/en-GB/apps/2f30fe76-c14a-46e9-a606-76ced27dd776 see the link for the watches this is compatible with. I have it on a Garmin 230 which are pretty cheap these days.
Note for the Data Field to show you have to be recording an activity ( run, cycle, walk, etc ). The same developer has also developed a widget to show the OS Grid Ref, the difference being you can see the widget when you are not recording an activity, eg if just walking around and you want your location. The widget is https://apps.garmin.com/en-GB/apps/07b03259-24c8-4828-b949-2e48ef04a2c5
You can have both the Data Field and the widget on your watch