Three reasons why the British fail to stop the Japanese attack on Singapore

1. The coordination between the troops and the Air force contingent in the region was poor, while the ground troops, particularly conscripts that came from India, lacked training and were not equipped properly. High ranking British officers, also lacked training in jungle warfare. In fact, some of them did not even considered that they needed to know how to conduct a war in the Malayan jungles, as indicated by some of their frustrated complaints that there was no place for them to train as the jungle was too big and it was bothering them, not allowing them to have enough training, the defense against the invasion down the Malayan peninsula was inadequate. There was also not enough food to feed the people thus the lack of food rationing lead to Singapore unpreparedness for the attack. "The water supply, Simson, said,would not last another 24 hours. The food supplies，he was told, were expected to last another few days while ammunition was running low and the only petrol left was that in the fuel tanks of vehicles." and Percival drafted his last message to Wavell: 'Owning to losses from enemy action water petrol food and ammunition practically finished. Unable therefore continue the fight any longer. All ranks have done their best and grateful for your help.'  from the book supports that British wasn't able continue fighting with the Japanese thus in the end they had surrender Singapore to the Japanese.
2. "The immense bureaucracy and the red tape of the civil administration and their lack of willingness to prepare fully for war that led to the huge suffering of the local population and the total unpreparedness for the Japanese bombing and attacks" shows that the British was not prepared for the attack from the Japanese, risking the lives of soldiers and citizens because of their unpreparedness therefore also failing to stop the attack upon Singapore
3. There were accidents where Japanese troops had captured British soldiers and actually sewed their penises to their lips before hanging them in trees where Allied patrols would find them; signs on their necks read "he took a long time to die". These displays were meant to and had successfully to demoralize Allied soldiers. "On the streets, large numbers of exhausted and battle-weary troops filled the city, their faces covered the mud, soot and grime, their bodies unwashed for weeks, their will to fight completely destroyed." from the book supports that the British had tried their best to fight but they had lost their hope and faith that they would succeed.

Sikh prisoners executed by Japanese troops, Malaya, circa Dec 1941-Feb 1942

British Army Lieutenant General Arthur Percival and his party carrying the United Kingdom flag on their way to surrender Singapore to the Japanese, 15 Feb 1942

http://ww2db.com/battle_spec.php?battle_id=47
Quotes from book: Singapore at War