The 55th Infantry Brigade, along with the rest of the 18th Division, was sent to Singapore in 1942 where it surrendered to the Imperial Japanese Army in the short but violent Battle of Singapore. The men of the brigade would spend the next three years as Japanese prisoners in harsh and degrading imprisonment.
Quoted from Wikipedia see link for a better understanding
"The Fall of Singapore, also known as the Battle of Singapore,[c] took place in the South–East Asian theatre of the Pacific War. The Empire of Japan captured the British stronghold of Singapore, with fighting lasting from 8 to 15 February 1942. Singapore was the foremost British military base and economic port in South–East Asia and had been of great importance to British interwar defence strategy. The capture of Singapore resulted in the largest British surrender in its history."
https://en.wikipedia.org/wiki/Fall_of_SingaporeI would be of the opinion that John was caught up in this conflict and became a POW after the 15th Feb 1942.
Given he was being transported in a POW transport ship September 1944 that would see John subjected to abt 2.5 years of captivity under very harsh conditions (Like many others)