Hi Hercules, we've been thinking of moving to Devon for ages and I have been applying for jobs for about 18 months. There are not that many jobs vacancies in my line of work, but the transition should be easier for you.
I have never lived in Devon but have been visiting the South Hams for 20 years and my 7 and 13 year old sons know Kingsbridge and surrounds quite well. I too worry how my 13 year old will fit in, and he was getting more reluctant to leave his friends and school. But we have just come back from a holiday and he is really keen to move down again. He's not much of a swimmer or surfer and wouldn't be persuaded to take them up. However he loves fishing and this hobby transfers well to Devon and he also loves playing cricket, so was very relieved to see that Kingsbridge has a thriving cricket club.
So it might be an idea to look at what your 12 year old likes to do at the moment and see how Devon can cater for his present interests.
I too am always struck by the lack of ethnic diversity in the South Hams. But I did notice more of a racial mix (yes, even in Kingsbridge!)on this holiday.
I do find the supermarkets and shops in general considerably more expensive in the Kingsbridge and the village areas of the South Hams that I know. I live in an area of South London where there's a large populatin of poor people and the shop/restaurant/cinema/swimming pool etc prices reflect this. I personally have a bit of a problem with the pervading comfortably off middleclass-ness of the area. I know the cost of living for us as a family would be higher
Another thing I'm considering is this - as jobs are scarcer in the South Hams than in London, if we move down, there's more chance our sons will be forced to leave home in their early twenties to get a decent job and may end up living far away from us. If we stay put in London, the chances are they will live closer to us when they reach their twenties. I know I can't predict the future, but when I think about moving for the sake of the family, I really wonder if in the long term, this is a false assumption.
Having said this, we are still very much looking to move.
As for the schools, there is a mumsnetter here who works at Kingsbridge school and whose children go to Ivybridge school and she knows the reputation of the local South Hams secondary schools really well. I hope she sees this thread. FWIW I have heard good things about both schools from our friends in Devon. When we enquired, Kingsbridge School is always oversubscribed.