That is if the information comes from all the way the front line to Pearl.
What might be more pertinent is that Winston may inform FDR of the British intent to interdict suspected Japanese invasion convoys heading to Thailand and that by itself may be enough to put the USN in the Pacific at a higher level of readiness. That would short cut the comms significantly.
Won't totally mean that Pearl is averted but it may at least be on alert.
How would this be different from the "War Warning" issued on 27 November? Pearl Harbor went on alert immediately after that; by 7 December, as there had been no action for ten days, the alert was relaxed. Such a report would be obscure, not definite. Churchill might pass it on, but not as urgent or requiring immediate action; he did not want to "cry wolf".
It's also not clear that it would get to Churchill immediately or during working hours, or that Churchill's message would get to Roosevelt immediately or during working hours. 2230 in Singapore is 1430 in London and 0930 in Washington, but it's Saturday. And 0230 in Hawaii - on Saturday, 6 December.
Which raises a question: the strike on Pearl Harbor was at 0700 on 7 December, which is 1400 in Washington, 1900 in London, and 0300 in Singapore on
8 December. The OTL Japanese landings in Malaya were at 0215 on 8 December. If the Japanese are planning to land on 8 December as OTL, would their invasion fleet be at sea on 6 December? Time zones make this very confusing.
From the Indochina coast to Malaya is about 400 km, which would be about 20 hours steaming at 12 knots. So to arrive at 0200, they would depart at 0600 the previous day (7 December). Which means, I guess, that they would be at sea on 6 December, moving to the assembly area.
I guess we have to remember that the Japanese did everything on Tokyo time (two hours ahead of Singapore time), and that in their orders and records, the strike on Pearl Harbor was on 8 December. So 6 December in Malaya is still over a day from action (on Japan's schedule). Gort has to be very careful not to launch MATADOR too early (or too late). Gort said "I would go Sunday morning...", that is, 0600 on 7 December, which would be about 20 hours before the Japanese landings.
It's about 100 km from the border to Singora, 150km to Pattani (along the road). Arriving before the Japanese would almost certainly provoke resistance from Thai forces. (OTL, Thai forces resisted the Japanese landings for hours, until ordered from Bangkok to stand down.) What Thai forces are stationed at the border or along the road?