I've got a list of coordinates in the UK Ordnance Survey grid system e.g. "NZ 27 41" or "NZ 284 449". How do I convert them in QGIS 2.10 to decimal degrees so I can map them in QGIS in WGS84? BTW, is there a way to deal with the spaces in those coordinates?
These are OS grid references, to convert them OS coordinates you can use 'ng converter' which is free from here:
Before using 'ng converter' you will first need to do a find and replace operation on the list of grid references to remove the spaces (easy enough in, say, a spreadsheet).
Load the output of the converter into QGIS as a delimited text file with the CRS EPSG:27700. From here the data may be 'saved as' an EPSG:4326 (WGS84 lon/lat) shapefile (or used as it is, converted to EPSG:4326 on-the-fly).
I spend a lot of time converting lat long coords, and I actually just do it in excel. It is a little bit slower but you have more control. If you can use the =mid() function to separate hours, minutes, and seconds, then you can use a simple formula to do the conversion. Then just load the excel sheet or csv back into QGIS.