From what I can gather on here, the closer you are to London, the more difficult it is likely to be to get a child into an independent school at an 'odd' time. Where I live, there is loads of movement into and out of indies (my daughter is at one, just gone into Year 10 - 2 children left at the end of Year 9 to go to different indies and 2 have come in this year from other local schools. That's on top of one who left to go to GS and one who has come in from abroad); and we looked at moving to the north of England not all that long ago and were offered places pretty easily in independent secondary schools. I think you should decide where you want to live and then go and approach some schools - it might not be as hard as you think to get a place. Good luck.
Thanks everybody for your inputs.
Amber: Is there a way to find out open positions in Independent schools outside of London? I would llike to chose the place to live depending on school availability. In all likelihood, my work place is bound to be within London but i am fine to travel.. Or will schools provide this information if approached?