Very interested to read this thread because I was about to start one on the same subject. (I am also quite anxious generally and get very worried when dd is off somewhere with friends)!
Gosh that is admirably early LadyMonica - my dh had the same experience as you (also lived in Norfolk) and then commuted to school in London from the suburbs by himself every day from about the age of 11 yrs. He virtually ran his parents business from about the age of 15 yrs. He is an extremely confident and self-reliant person now. I had a much, much more sheltered upbringing and am overly cautious as an adult I think.
DD (nearly 14 yrs) looks quite mature and can be both very responsible and independent and rather naieve, all at the same time! I know that is the nature of adolescence but it is terrifying because you never know which side of them is going to emerge in any given situation.
DD usually travels to school on the underground tram/metro/bus to school and back by herself and with friends, she pops in to shops by herself and with friends, she goes to our local shop (5/10 min walk away) to buy something by herself, walks to her nearest friend's house by herself (10 mins) she operates an on-line lawn-mower/mini-tracker for half a day independently, she goes hacking with one or two friends on horse-back, goes off on (adult supervised) trips and camps with friends quite happily, but I am not sure she has had enough independence doing things on her own ifsywim.
I am desperately wracking my brains and looking for ways to give my dd more independence but within safe boundaries (bearing in mind we live in a capital city (not London) that has had quite a few incidences of terrorism recently). I'm a bit stumped how though to be honest. Maybe volunteering might be an option?
Having said all of the above, I firmly believe that sometimes you simply must leave them to it to work out things for themselves.
DD (and her friends) can be a bit lazy sometimes (it sometimes seems as if they go through life in a semi-dream, plugged in to Iphones or whatever, being ferried about here and there) and they will happily let all the responsibility for travel arrangements (for example) fall on parents if they are hovering around and then they don't even have to think or properly engage about the logistics and get down to the nitty gritty of travel ie when is the date, where are we going exactly, how do we get there, how long will it take overall working backwards from the start point allowing for delays, how do I access the train timetable, how much money do I need etc etc?
They won't even take proper responsibility for tasks at home such as cooking or washing, and they will never think about them properly, if us parents are hanging about and available to ask all the time and problem solve for them.
It's an incredibly difficult balance to strike though. I will be very interested to read how other parents are tackling this.