Pythagoras

Having whetted your appetites (or not) by an earlier mention of applying trigonometry to grid references, here's what you do:-

Using six figure references take the first three digits of one reference from the first three of the second and square the result. Do the same with the last three digits and square that result.  Add the two squares together and take the square root of the sum.  This gives you the straight line distance between the two points in multiples of 100 metres.

All together now... The square on the hypotenuse is equal to the sum of the squares...

As I believe we say nowadays ~ simples.