[Observer1940]

A WW2 six figure grid reference (preceded by a letter) should be reasonably accurate, if the correct 1" to 1 mile Ordnance Survey RAF / War Office "War Edition" grid Map is used.

6 Figure to 4 Figure
For example U.892361 expressed in four figures would be in the U.8936 Square on a four figure map. On a four figure map you have to sub-divide the square into tenths, to plot a six figure reference, but I expect you learnt that in Geography.

Pre WW2
I have one of the pre-war (pre WW2) OS 1" War Office Sheets for the AA and they used the same grid as the later 1940 RAF War Edition WW2 Maps, I noticed.

Crash Expressed as Mile/s NE of ...
An initial report may just say 1 mile N.E. of Southam for example, some of these were probably only accurate to within say a quarter of a mile, or occasionly less.

Seventh Series 1" OS Grid Different From War Edition
However, I have seen a posting where someone attempted to use a post war Ordnance Survey National Grid Seventh Series (7th Series) 1" to 1 mile Map (Somtimes called the 1" OS Red Series, as the covers are coloured Red). However comparing the two, the letter/number grid is different (has been offset / moved) over the later 1" map.

Unfortunately, I don't have the War Edition Sheets covering Kent, but if you have the reference and place, next time I'm at TNA, I will see what I can find out, unless someone else answer's you first.

Lucky
You are very lucky to get an official WW2 Flying Accident Report (RAF Court of Inquiry / RAF Court of Enquiry)!! The MoD inform me that they have many thousands (20K) uncatalogued from WW2 in 2010 and we can't get them to search them fully under Freedom of Information Act, as it would exceed the FOI limit. The sample search under FOI revealed they were injury flying accidents according to the Judge in my recent challenge to Tribunal.

Regards Mark