Interesting comments about lat/long versus grid coordinates and ease of obtaining / converting.
In my opinion, the absolute crux is to know in advance what you are going to use, to be practiced in your chosen method and to know exactly what it is. More on that at the end of the post.
I prefer grid coordinates as that's what I've used for navigating on the hill since I was taught that in the scouts. I've only had to call the emergency services out twice and grid coordinates were accepted on both occasions. Once in a mountain rescue situation, and one in a burning car on the road situation.
Knowing what method you are using, how hard can that be?
Well, there are two methods of expressing a grid reference., something I only learned recently. The method I was taught donkeys years ago when using paper maps is of the form SK 320 085 where SK refers to in which 100 Km square the reference is, then the other two groups of 3 numbers are the eastings and northings read off the edge of the map. This isn't meant to be a tutorial, by the way, I'm just presenting enough detail to make my point. So, the numbers 320 085 are a reference to an origin that is the bottom left corner of the 100 Km square, SK in this case. However, another form of grid reference exists that is all numeric, where the origin is the bottom left corner of the SV square, irrespective of which square you are actually in. I only found out about this when somebody showed me how to obtain a grid reference from the streetmap.co.uk website. Streetmap quotes all numeric first. The all numeric reference can be determined from a paper map, by the way, there is a prefix number that can be found in the corners of the map.
Lat/long can be expressed in degrees, minutes and seconds, or degrees and decimals of a degree. And I'm really not sure whether the lat/long values marked on an OS map correspond to WGS 84 that I understand is used by GPS systems.