OK, except that Britain wasn't inflexible, it was prepared to recognise "Japan's special position" in China - to a certain extent at least, iirc - during the 1930s and try to work with Tokyo.That doesn't buy Britain time though. They have interests in the far east and making Japan stronger helps exactly none of them.
But even if we take your position, an undistracted Britain would be a greater danger for Japan, and if Southeast Asia was reinforced, the Japanese might think that it would be better to attempt a "Pearl Harbour" like operation against British forces first - if they really need to take over Southeast Asia.