Foreign air units, friendly or otherwise, may not fly over the territories or land on the carriers of a power that is neutral (not at war with anyone). A neutral power’s air units may not fly over territories or land on carriers that do not belong to it. Allied air units belonging to powers not at war with Japan may not fly over original Chinese territories (regardless of who currently controls them). No air units may fly over neutral territories, unless it is to attack them. Any other movement of air units is allowed.
To answer Jmite’s specific question, a Japanese plane may fly over a UK territory (and vice versa) in either combat or noncombat movement before they’re at war with each other, because neither Japan nor UK is neutral. Japan is at war with (at least) China, and UK is at war with Germany and Italy.