I found one online tool but it only calulates the distance between the first bit of two postcodes (so from SN13 to SN12, say) which doesn't really help - particularly as we're at one extreme of the area and the school I'm trying to convince dp we're too far away from considering is at another.
Google Maps seems only to give travel distances - which can be far greater than the direct line distance.
Or should I just go straight to the LA? Would they tell me?