Ok...here goes with a scenario to answer the OP. These details were gleaned from original files at the UK National Archives. I'm working from memory but I have all the references once I locate the hard drive that is the back-up for my MA thesis.
The key challenge for Brooke-Popham was time. He needed time to implement Operation MATADOR but waiting until the Japanese landed in Malaya wouldn't afford sufficient time. Equally, Brooke-Popham couldn't attack the Japanese troopships before the landings commenced because that would be contrary to the Hague Convention of 1907. Britain had been courting Thailand, attempting to persuade Bangkok to ally with Britain against Japan in hopes that MATADOR could be launched pre-emptively....but without success. Brooke-Popham needed incontrovertible proof of Tokyo's intentions before the Japanese forces started landing, preferably in a way that could be shared with Bangkok to enable MATADOR to be launched in time....and hunting for Japanese convoys in the Gulf of Siam would NEVER provide the necessary warning or evidence.
In early 1941, Brooke-Popham identified a set of key intelligence indicators that would warn of a Japanese attack. The most important of these was the arrival of Japanese fighter aircraft in the region. Bearing in mind that Japanese forces were already established in French Indo-China, it was logical to assume that any deployment of fighters would be to that country. Thus a key task for the Far East Combined Bureau (FECB), Singapore's intelligence analysis organization, was keeping tabs on airfields and the air order of battle in French Indo-China. Now, in reality, the IJA only deployed fighters at the last minute, just a few days before the invasion forces landed on the east coast of the Kra Isthmus...however, a few days would still provide much better warning than a message saying that landings had already begun.
One of the Assistant Consuls in Saigon was a man named William Meiklereid. In late-1941, Meiklereid sent a report to London and Singapore about Japanese airfield construction activity in French Indochina. One of the most active developments was the creation of a brand new airfield on the island of Idu Phu Quoc which was, geographically speaking, the closest point in French Indochina to the eventual landing points at Singora, Pattani, and Kota Bharu. Meiklereid's report provided a sketch map of Japanese aircraft dispositions and the locations of airfields that were under construction. Although the report is undated, it almost certainly arrived in Singapore before the commencement of hostilities.
British expectation was that the Japanese invasion convoys would land in French Indochina before embarking on a northward overland invasion into Thailand and around the northern shore of the Gulf of Siam to reach Malaya. However, the development of airfields on Idu Phu Quoc pointed to a different course of action. If the Japanese intended to push north then they would have expended their energies building airfields in the north near the border between French Indochina and Thailand. Instead, they built airfields to the west on an island. Now, it's a basic tenet of air defence that you put your fighters up-threat from the locations you're trying to defend. If Japan intended a move northward into Thailand, surely their fighter aircraft would have been aligned in the north? However, if Japan's intentions were westward facing, then fighters could be expected at the new airfield on Idu Phu Quoc. Brooke-Popham should have been watching that airfield like a hawk. Unfortunately, either FECB dropped the ball or Brooke-Popham himself started over-obsessing about the location of the invasion convoys.
Photographic reconnaissance assets were available in Singapore that could have kept watch on Idu Phu Quoc. Two Buffalo airframes had been converted for the PR role, one a "short range" (SR) airframe with a single camera (serial W8136) and the other a "long-range" Buffalo with extra fuel and oil tanks, and a pair of cameras providing stereo imagery (serial W8166). Development of the LR Buffalo was problemmatic and it wasn't ready for operations until after the Japanese invasion. However, even the SR Buffalo had sufficient legs to reach Idu Phu Quoc from airfields in Northern Malaya. A further PR platform was developed by adapting a Bristol Beaufort. Some 6 airframes had arrived in Singapore in November 1941 from Australia, the first of a planned replacement fleet for the Vildebeests of 36 and 100 Sqns. Given the deteriorating political situation, and teething troubles getting the Beauforts into service, it was decided to send 5 airframes back to Australia but to retain one for PR purposes. It, too, had sufficient range to reach French Indochina, indeed it was tasked with just such a PR mission on 6 and 7 Dec 1941...unfortunately, it was tasked to photograph ports and harbours rather than the new Japanese airfields.
If FECB and Far East Command set up a dedicated imagery collection effort against Idu Phu Quoc, it's likely that the arrival of IJA fighter aircraft would have been detected days before the Japanese invasion of Thailand and Malaya. The photographic evidence of the deployment could have been provided directly to the Thai government in Bangkok and, through embassy contacts, to the US. Such solid evidence of Japanese intentions could have forced a change of heart in Bangkok, and an invitation to British forces to help defend the key port at Singora....essentially enabling MATADOR before the Japanese invasion. Sadly, the fleeting opportunity was missed and the rest is history.
However, the above is a viable option for Brooke-Popham to respond more effectively to the Japanese threat, and it employs resources that were already available to him in November 1941.
