While it is nice to speculate what might have been in the Churchill message on 26 November 41, British records show that there were no JN-25B decrypts prior to 7 Dec. 41. OP-20-GYP-1 records show the same for the U.S. Navy. Actually, no JN-25B message ever spelled out the attack on Pearl Harbor. Thus, if there were a tip-off on Pearl Harbor in the Churchill message, where did it come from? You seem to have avoided that issue. Some conspirator theorists have speculated the Sorge ring obtained such information, but even if true that would have gone to Stalin not Churchill. Thus, I would suggest you hold off speculating what might have been in that message and find such information within the British government at that time.
Where did such tip-off information originate? How did it get to Churchill? If you can provide documentary evidence of such a source of a Pearl Harbor attack tip-off and it's transmission to Churchill, then you might consider voicing your blue sky speculations as to its inclusion in a certain message. Mind you there is no evidence of such information ever being received by Roosevelt either.