I'm going the same way with my world war 2 TL, and I think successfully defending it against the Japanese would be one of the most crucial steps to making this happen.
The quick IJA victory essentially destroys Britains reputation in the Far East as a military power capable of defending its Empire. I think the more prominent nationalist groups gained a huge surge in support after the end of the war. This could prevent the rise of the Labour Front and thus the Merdeka Talks from taking place.
This link has a little more information
http://en.wikipedia.org/wiki/Merdeka