You should see the place structure as levels, with parish the special one (being a level in a religeous setting). So the labels are the US labels, but translaters can just change them to the setting of their country.
There is no en_UK language pack for GRAMPS though, so no street, district, city, county, country for you. You could easily make a en_UK language file with this for your system though, which means you keep the usa language file and only change these strings (http://www.gramps-project.org/wiki/index.php?title=Translating_GRAMPS
For the future we have on the features the ability to select the country and have the labels of the field update to the naming in that country. No developers are working on that at the moment though.
You mean this: http://en.wikipedia.org/wiki/British_national_grid_reference_system
Does not look usefull outside of the UK. Conversion to Lat/Long is difficult too, while that is important for future integration that is planned with openstreetmap/google maps.
You have some options:
1/misuse the lat/long field to enter your data. Like that you have your data stored, but GRAMPS cannot use it for map display
2/Use a note or the internet tab (with page of http://www.ordnancesurvey.co.uk
) to document your ordance survey. Do the effort to convert to lat/lon so you can use map data in GRAMPS
3/do feature request to add this as a field to GRAMPS. Obviously the field would need to be optionaly as only UK people have interest in it. A developer would have to code it, as it is only usefull for UK this might not be a high priority.Alternatively one could add attrributes tab to places, and store this as an attribute of the place.