Ladies! Please!
Arsenal mum, your private school is unfortunately lagging behind national developments, in that the new national curriculum the whole life of the child and school is emphasised:
http://curriculum.qca.org.uk/
This is slowly but steadily replacing the narrow interpretation of the subject-based national curriculum. In your son's school's case, I am guessing not many children are following a similar course of excellence so they are doing the good old jobsworth thing of ignoring it till it goes away. If he had been signed on to a rugby union club I expect their reaction might be different but sadly soccer is seens as rather infra-dig by the ruling classes.
There are definitely secondary schools and academies with specialist status for sport, all over London and the whole country, but you will be moving away from a possibly sheltered, small-scale environment into a larger scale one where individual care may not be of the same intensity - then again, might be excellent. Get yourself down to a range of seconary schools (include faith schools) in a commutable area and see how they strike you as a prospective family. Good luck to you and the budding champ!