Actually my sister's son's primary school in Fulham prefers them to walk on their own as they ar more responsible and aware. It is when they are with a friend that they are more likely to get distracted and do silly things! Obviously depends on the children, but interesting that they have a different take on this to someone's post above.
I'm currently spending some time in France and they are allowed to walk to or from school with parental approval from the age of 6, which I thought was quite young even if you live a few doors down... Although I guess they are not encouraging it from that age, it is more that it is not allowed younger than 6. I'm enjoying letting my 11 and 7 year olds walk to/from school together here, which I wouldn't do in London, but the journey here is very short and safe.