The United Kingdom had already been fighting the Germans for over two years when the Japanese attacked Pearl Harbor and the Dominion garrisons in Malaya, Hong Kong, Shanghai, and elsewhere. The United Kingdom, under Winston Churchill, had spent a year fighting Germany and Italy alone. The war had caused the British armed forces to focus on the immediate problem of the European war, leaving their Pacific possessions without enough soldiers, aircraft, tanks, guns, warships or training.
The loss of the new battleship HMS Prince of Wales and HMS Repulse, shocked Churchill and all of the Dominions. The Royal Navy would fight actions in the Java Sea and the Indian Ocean, but would not take up offensive operations in the Pacific until November 1944.
Particularly hurtful to England's war effort in North Africa was the immediate withdrawal of Australian troops, who were rushed back to defend against a supposed invasion of Port Darwin. These troops were sent to Singapore, and were captured with Percival's surrender.
The loss of Singapore was the worst defeat in British history, with hundreds of thousands of troops entering captivity and many dying. The full scope of the Allied POW treatment at the hands of the Japanese was not fully known until after the war.
The immediate effect of the Japanese attack was to bring America into the war. As set out in the Atlantic Charter of 1941, Churchill and Roosevelt wanted to deal with Hitler first and then the Japanese. Circumstances in the Pacific prevented most of the American forces from being deployed to Europe until late 1943.