Not Wansworth but a friend wanted to dipute distance to school in our local area. It will be measured over the shortest safe walking route. We went out with a GPS device and found the LEA's distance unv=cannily accurate.
When we set up a 'walking bus' for primary school someone from county came out to walk the routes with us and do a risk assessment. Maybe speak to your LEA to find out if risk assessments have been done on unlit areas? Danger is usually seen to be from traffic so you may be disappointed if you're objecting to a public path over a common or park.
It goes past a cafe, 2 play areas various other municiple facilities and is a recognised cycle path too!
(as an aside interesting that they assessed it for safety wonder if there is any come back if there is an accident/robbery? With current behaviour I'm surprised they would agree to do that but the walking bus thing might make it different)