In practical terms - On the CAA charts (Lamberts Conformal Conic Projection) measure the distance betwixt the two points with your dividers and then read off the measured disance using the Latitude scale (use the latitude scale alongside the approx. area of your journey)
And just to confuse matters .....
In Navigational terms: You can use a Haversine formula to gain the great circle distance - Thus :-
Hav(Dist) = Hav((90-LatA)~(90-LatB))+Hav(longA-LongB) x Sin(90-LatA) x Sin(90-LatB)
If you want to use Excel to help the sums for you you could use:-
60*SQRT((LatA-LatB)^2+(COS(((LatA+LatB)/2)/RadConv)*(LongA-LongB))^2))
(Where RadConv = 180/PI()) to do the radian/degrees conversion)