In the hut the soldiers' clothes become dry very quickly. Even when Laurence Harvey is wringing his shirt to get the water out, the rest of his clothes are dry. In the jungle during the rainy season, clothes would take hours if not days to dry out.
The Japanese had a LMG Type 99 which was very similar to the British Bren. Not many Type 99's survived so I suspect they used a Bren as a lookalike.
When the Japanese are firing their machine gun at the patrol it is quite clearly a British Bren gun with the characteristic vertical curved top loading 30 round magazine. The Japanese had MGs with short horizontal stiffer bullet clips that fed in from the side.