They use a system called Ordnance Survey Address Point, now called Address Base. It is only available commercially, so you can't access it as a member of the public.
There are some reasonably accurate free tools around - this one for example: http://www.freemaptools.com/distance-be ... tcodes.htm
Alternatively, a 1:25000 OS map and a ruler works quite well! (The distance is now measured as a straight line, not as walking/driving distance.)
She could call the Admissions team to ask them to calculate the distance, but frankly it would be pretty pointless because no one knows what this coming year's allocation distance is going to be.
Which school is she hoping for? Some of the schools have expanded their Y7 intake for 2016 and that will increase the distances for them.