Store both geometry and geography or store one and calculate the other?
To elaborate a little more, the database will be populated with pre-existing data that contains both British National Grid (EPSG:27700) (OSGR) and converted WGS84 (EPSG:4326) values, and there will be a regular update of that data from external sources. This would argue for two columns, because I don't need to do the calculation.


You can always reproject with ST_Transform (though you may have minor rounding errors). I would convert and store in WGS84 explicitly and reproject to EPSG:27700 when you need those advantages. The geography features are really useful, and it makes life massively simpler to have them.

Not to mention, you can indexes to cover that conversion too if you frequently query on it.