So the premise of your question relies on a chain of inferences of which each link is very questionable. Firstly that the British SIS knew details about UN battle plan. Secondly that the Soviet agents personally knew those details. Thirdly that they chose to pass those details to their NKVD connections. Fourthly that the NKVD believed this information. Fifthly that the NKVD would think it important enough to pass to the military forces in North Korea (either the North Koreans themselves or soviet advisors) and Lastly that the information would be considered reliable by military leaders to change battle plans.

Just starting from the top, detailed battle plans like time and location for an invasion are not shared very widely. In particular, SIS might not really have need to know, since they would have a mandate to gather intelligence about the soviet bloc generally rather than a specific warzone. War zone intelligence might be the purview of the Army or Navy intelligence units.

After that, information is compartmentalized within intelligence agencies. If a specific person doesn't need to know something, they won't be told it, even if their rank is high. Seeking information a person isn't entitled to is a breach of protocol that'd be very dangerous for an agent. The Cambridge ring's main priority in intelligence was exposing British attempts to gather intelligence on the Soviet Union so they might not choose to seek military intelligence or pass it along if they knew it.

Furthermore, if they had communicated it the NKVD might chose to sit on it, in order to protect that intelligence asset rather than risk compromising it. In the Stalinist Era, the top priority was preventing the penetration of the soviet state by foriegn agents, not expanding communism abroad. Also, the NKVD was not primarily charged with Miltary intelligence, which was a function of a separate Soviet agency, the GRU.

There would have been a lot of other ways for the GRU and North Korean intelligence to gather intelligence about Inchon preparations. There was an active Communist Party in Japan at the time (where Inchon was staged from) and there would have been Korean agents within the South's armed forces. Furthermore, there aren't that many suitable harbors where a landing could be made, and the tidal tables meant that there were only a few times when a landing would be easiest. It is likely that the communist forces in North Korea could have guessed the landings would come when and where they did even without any intelligence penetration by foreign agents.

The North Koreans could not have guarded against the landing without withdrawing forces from their final assault on the Pusan perimeter. They may have simply taken a calculated risk that the perimeter would collapse before any invasion could be launched. Choosing which intelligence to believe and which to ignore is difficult for any general.

Now comes the difficult question of "how do we know" which is always a very difficult point in the history of intelligence. We have very limited access to the Russian, Korean and Chinese intelligence history. Nor is the historiography of the Korean War in the west very rich in North Korean sources, even the best western historians have to rely on interpretation to determine how the North Korean leadership planned and what they knew. On the Soviet side, a portion of the KGB (and NKVD) archives were compromised in the 1990s and published in the west as the Mitrokhin Archive, but one can never be sure about the extent of information available. As far as I am aware, there is no evidence that the Cambridge ring knew any Korean war intelligence, but it's really hard to prove they did not!

I'm not sure your estimation of Eastern Bloc access to operational military secrets is necessarily correct — at least, not via the agents you mentioned or at the times you're inquiring about.

For one thing, neither Burgess nor Maclean were intelligence officers at the time of the Korean War — both were Foreign Office diplomats. Philby was indeed an SIS officer — at the outbreak of the Korean War, he was stationed in Washington DC as the senior British liaison officer to the US intelligence community, but he was recalled to London and resigned from SIS in July 1951.

It does appear that Burgess and Maclean provided some useful political intelligence in the runup to the war. But they both defected to Moscow in May 1951, so the overlap with the war itself is relatively limited.

At this point, Maclean was working in embassies in North Africa and the Middle East, and in the Foreign Office in London. Burgess was working in the Foreign Office's Far Eastern Department until August 1950, and provided some important intelligence to Moscow during the period of escalating tensions. This is from Christopher Andrew and Vasili Mitrokhin's The Sword and the Shield: The Mitrokhin Archive and the Secret History of the KGB:

The impact of Burgess and Maclean's intelligence in Moscow was heightened by the outbreak of the Korean War in June 1950. Maclean's deputy on the American desk, Robert Cecil, later concluded that the Kremlin must have found the documents provided by Maclean "of inestimable value in advising the Chinese and the North Koreans on strategy and negotiating positions."

There's some more detail about Burgess and Maclean's Korean War-era work in Andrew and Oleg Gordievsky's KGB: The Inside Story of its Foreign Operations from Lenin to Gorbachev:

[In the Far Eastern Department] he was able to keep Moscow informed on the detailed formulation of British policy to the People's Republic of China, founded in 1949, and toward Korea in the period leading up to the outbreak of war in June 1950. Though only a grade 4 officer, Burgess had regular access to intelligence assessments from the Joint Intelligence Committee, the War Office, and General Douglas MacArthur's headquarters at Supreme Allied Command in Tokyo. Of particular interest to Moscow Center would have been a minutely detailed military intelligence analysis in April 1950 of "Russian assistance to the Chinese communist forces," which effectively revealed all that Western intelligence had gathered on that subject only two months before the outbreak of the Korean War.

But from August 1950, Burgess was reassigned to Washington, and although he still provided information on Korea to the KGB, it was even more decidedly political rather than military intelligence. Back to KGB:

In December [1950] President Truman gave an ill-judged reply to a question about the use of the atomic bomb in the Korean conflict: "Consideration of the use of any weapon is always implicit in the very possession of that weapon." Clement Attlee hurriedly arranged a trip to Washington to discuss with the President this and other policy issues of the war. Maclean was able to pass to [his KGB controller Yuri] Modin both the briefing papers for the visit and the cabinet paper reporting its outcome.

The other thing I think you have to consider is that intelligence-sharing relationships between allied countries are always somewhat tense. So it's not certain that such strategic intelligence as the Soviets succeeded in gathering would necessarily have been communicated to Beijing or Pyongyang in a timely manner. It's also important to note that both the PRC and North Korea were nascent states at this point, without fully-developed intelligence communities.

This is the Andrew & Gordievsky appraisal of the early years of the Sino-Soviet intelligence relationship immediately prior to and during the Korean War:

On the founding of the Chinese People's Republic in October 1949, the MGB [the predecessor agency to the KGB] had proposed sending a large group of advisers to China and invited the Chinese to send some of their own intelligence officers to Moscow for training. Mao Zedong accepted both suggestions. From the outset, however, the Chinese were determined that their intelligence services should not, like those of Eastern Europe, become controlled by the MGB. Though anxious to learn from Soviet tradecraft and technical expertise, they rejected MGB intelligence manuals as unsuited to Chinese conditions. MGB officers were also not allowed, as in Eastern Europe, to take part in Chinese intelligence operations.

Just on the Inchon question specifically: there does appear to have been some Korean access to intelligence about Operation Chromite, but it's not clear whether that intelligence actually made its way back to Korea. This is from Assault from the Sea: The Amphibious Landing at Inchon by Curtis A. Utz:

FECOM worked even harder to keep the true destination of Admiral Struble’s task force secret. With men, supplies and ships concentrating in the ports of Japan and at Pusan, there was no way to hide the fact that an amphibious operation was about to take place. So widespread was the speculation that the press in Japan referred to the impending landing as “Operation Common Knowledge.” Confirming MacArthur’s worst fears, in early September, counterintelligence agents uncovered a North Korean-Japanese spy ring. When the leader of the ring was arrested, he had a copy of the Chromite operation plan. No one knew if he had been able to transmit the plan to Pyongyang.