The Brits have long claimed that they knew Imperial Japan was going to try and take their colonies and territories but they have long denied that they had any intelligence about Pearl Harbor. Japanese commanders not involved in the attack were also kept in the dark from what I have read although they were given warning orders to prepare to take objectives.
Japan was a very closed nation to the rest of the world and its difficult for us to comprehend in today’s age and technology not to mention that while both Britain and the US cracked some of the Japanese codes they were radio silent after they sailed from Japan to attack Pearl Harbor.
The American Navy at the outset of WWII was a mere shadow of itself two years later. Japan gambled that we would curl up in a ball and stay home. They chose poorly. By the end of the war our Pacific Fleet was even more powerful than the Russian Army in terms of dominance of the battle space.
Our submarines literally ran out of shipping to sink to the point that they were mostly destroying junks and fishing boats with cannon fire. Our aviation assets had total dominance. The Imperial Fleet was destroyed and Japan could no longer contest the airspace and focused on suicide attacks.
I believe they had one capital ship left in harbor that was damaged. They lost well over 300 warships and over 300,000 men in their Navy and that does not count the total decimation of their merchant fleet and even larger loss of men. They were incapable of replacing much of anything by 1944 and the loss of their experienced pilots was just as devastating as the loss of material.
It was in whole one of the most dominant defeats in the history of man if you view the war as one event. Decimation does not even begin to describe it and it was going to be even far worse as new planes, carriers, and ships continued to be built up to the atomic bombings.
Absent the atomic bombs and the quick surrender the siege of Japan would have been one of the ugliest chapters in human history. A horrific ending to the most horrible event in human history that would have made the atomic bombings look like a blip of suffering.
As I said the British, an island nation and resource people in that in their history had to learn to make do with little and were very effective at guerrilla style, hit and run/commando warfare so as I said intelligence had to be guarded and shard judiciously with even their allies. As Napoleon said "No nation has allies, only interests'' and Britain's interests, well were Britain. Yes the British certainly were out to maintain their colonies which had been their economic life line of trade and goods and they could only do this by maintaining a powerful navy. But the Royal Navy couldn't be everywhere . As Clausewitz said "He who endeavors to defend everywhere defends nothing'' Churchill knew this and he knew Nazi Germany was Britain's main threat. In his memoirs Churchill confessed that his greatest fear was the U boat threat. He knew that if the Germans succeeded in sinking enough shipping to Britain they could quite likely succeed in starving Britain into stalemate or surrender so it was understandable to get us into the war even if it meant knowing the Japanese were going to attack but they saw that as their saving grace, if you will. Churchill later admitted that despite the carnage of the attack he said, to the effect "Now that America, this colossus of 130 millions souls and industry was in the fight with us I could now sleep the soundest of sleeps''.