The most accurate (to a 3 metre square) and now most common reference used by emergency services is the what3words app.
A blue dot and the 3 words at the top of the screen signify the position of the phone.
NB – Save the 3 words before you leave the incident if you need to move away before making the call.

Alternatively, using Google Map, a blue dot shows your position (you do not need to download a map).
Touch the screen as near to this as possible and a red position marker will appear.
The coordinates for this red marker are shown in the search box. 

GPS devices & Mapping Apps (e.g. Memory Map using "GPS lock") will provide grid references.
Check how to use this function if you have not already done so.