I guess it depends where you live, and how far away the aeroplane will be diverted if need be.
Personally I wouldn't put my child unaccompanied on a flight because I live on an island, so if in the event that the plane can't land here they can end up somewhere that I won't be able to get to and they will have to stay in a hotel with a stranger (cabin crew).
If you live on mainland uk this is less likely to be a problem as you will be able to get to, say, london from birmingham reasonably easily.
It isn't a problem for the vast majority of flights though.