FTO near London
Think your best bet is Wycombe Air Centre, you can take the Chiltern line from London Marylebone (first via tube on the Bakerloo line) and then it is a short bus journey or taxi from High Wycombe station to Wycombe Air Park.
I don't know about the "South" side as as far as I know the only only possibility is Biggin Hill, but I don't think there is an FTO there. Next closest is Shoreham and then Bournemouth on the southern side.
Hope this helps. P