I've now spent quite a few hours playing each of the four countries on Immortal level. It's a lovely design and well-balanced in my opinion, but the CIV4 AI is so poor that it really detracts from the quality of the scenario. I commented before about the Americans easily being able to hold Manila. The same holds true for the Brits and Singapore. Both of these should fall to retain historicity, but any reasonable defense holds them. As a test I set myself some handicap rules playing the Brits (on Immortal) ... only use air and naval power in local theater until more is built there (so SE Asia has just the 3 P40s and 1 Bo, plus the 2 Cr from south, 1 Dest, 1 Sub). Also, for the first two years build no military units in Canada, just improve research and economy. Otherwise, building and flying in planes makes it way too easy.
Even with these constraints it's still easy to hold Singapore. This is the fault of the CIV4 AI, not the scenario. The AI has limited concept of combined arms, is seemingly incapable of making an amphib. attacks (crucial in a Pacific scenario), and picks dumb bombing targets. If you watch it trying to go after KL, for example, it does not bring in naval bombardment and half the air bombing missions are to destroy buildings or production (which you should almost never waste time doing, btw, especially not a city you are sure to capture). When it finally reduces the defense (which is unnecessary - just hit the units), it attacks in a poor order, wasting valuable artillery on direct attack instead of ranged bombardment and often losing several pieces. The AI seemingly has no concept of the importance of attrition, protecting and promoting units, so it also "hangs" lonely units in vulnerable spots where you can weaken and eliminate them.
So, it finally takes KL (which you can even prevent with a little finesse) the Japs get that big force NW of KL on the next turn ... 5 tanks, 3 Nip. Inf., 4 Combat eng., 2 Art. If only they would attack Singapore! But no, they mostly saunter off toward China, unless you already took Bangkok in which case athey hole up in KL and never march on Singapore. Sometimes they move a few infantry one step toward Sing., you ping with the artillery and a ship bombard and they duck back. So, an idea on this ... how about if you put that force SE of KL right next to Singapore? Maybe then they would do what they are supposed to do and attack. I'm not sure they would as they might wait to get all the defenses down, which by then is hard since the Jap bomber force has been reduced. Sometimes a naval force comes in, but it sits there by Sing. and does not bombard. Likewise, there are many cases of artillery being in bombardment range of good targets and not doing anything and as mentioned the AI tends to see artillery as attacking pieces as opposed to using their for bombardment.
There are many flaws in the CIV4 military AI, and no doubt there must be a forum on that topic somewhere. Some things would be hard to program, but a few things seem doable. If they would just stop hanging pieces and try to keep ships in bases most of the time and concentrate on attrition and protecting and promoting units that would help. When attacking a target city never waste a bomb run hitting buildings, production, or surrounding improvements (not worth the risk when there is flak or CAP). Use naval forces (and/or artillery) to reduce defenses and air to weaken the units. If there are few units, just concentrate on them and forget the defenses. Pick a sensible attacking sequences and start when the best attacker (armor or ground) is preferably over 90%, though sometimes less. Etc., etc. - obviously to anyone who has played a while, but totally alien concepts to the AI.
It's also worth noting that when you step up in "difficulty" level that only seems to affect relative production and research rates. I've never noticed any meaningful improvement in strategy or tactics, though there may be a few minor things.
One last suggestion that does not have to do with AI. Is is possible to put label names on capital ships that would show above or below the unit on screen? It's just a cosmetic thing, but you've done all that research to make the units accurate to history so labeling some of the carriers, battleships, and cruisers would be a cute touch (or let the player name them). Imagine, for example, when the American task force pops up in the slot near Guadalcanal if the carrier were labeled "Enterprise" or "Grey Ghost," and so on.
I started a discussion a few months ago about "Making the AI More Aggressive" but nobody helped out too much, instead just pointing out what needs to be modified, but not how to go about it to get better results. 
