It should be noted that the 'line' formation that supplied the main forces of the invasion, the 38th Div (consisting of 228, 229 and 230 regiments) counted roughly 25000 men only. And it was also formed as a garrison unit in 1939, responsible for garrison duty around Canton before the war. It was claimed that they trained ~9 months for night attack and infiltration tactics for taking the Shing Mun Redoubt, but qualitatively they are not that superior to the Garrison.
25000 Men? Bloody hell that's almost twice the size of a European infantry Division
And the British opposed them with 5 Infantry Battalions - each with about 800 men of which 450-500 are in the fighting companies
Its pretty clear to see that the Japanese held a massive - insurmountable superiority in numbers.
And while the 38th Division might not have been a veteran formation - the IJA had been fighting a bloody campaign in China for some years so many of the Officers, Specialists and NCO's very likely had campaigning experience.
After the Battle of Hong Kong the same division also provided around half (~24000) of the invasion force to the Dutch East Indies, and later on Guadalcanal on where the division was broken. The Japanese were operating on a shoestring that relies on their operations going smoothly one by one.
I'd say providing 2 Infantry Brigades and the manpower for an Artillery Brigade (if not the majority of its guns) to the Malaya or Burma theatres would have delayed other IJA units and derailed other parts of their time table.
And this 'time table' is only something that we know after the event so should not be a consideration - had Hong Kong been a real threat to Japanese ambition and had a larger or more effective defence delayed the fall of the Colony then the Japanese Army could simply have decided to call off the attack and besieged it with perhaps 1 of the 38th Regiments freeing up the remaining 2 for service elsewhere much sooner.
The Defenders in Hong Kong simply wither on the vine.
The Allies could afford to lose Hong Kong - it gave little strategic advantage to either side.
Malaya and ultimately Singapore on the other hand........
It is also problematic that the higher-ups were split on defending Hong Kong or not. The result was an under-strength garrison that tried to do what was planned with more in mind and failed. The original defense planed called for (at least) 2 brigades and that strength was only reached with the arrival of the Canadians, which caused the 2 Royal Scots and the 5/7 Rajput to be shifted to the Mainland to bolster the 2/14 Punjab (responsible for the center of the line) without much preparations.
This here is the crux of the issue for me.
To successfully Defend Hong Kong (and I'm not entirely sure what that 'Victory' gains the Allies) required far more troops than the ones provided.
More troops were not available so its clear to me and I am sure that even without the benefit of hindsight it was equally clear to those in Whitehall at the time that the attempt to defend the Colony was doomed.
I'm afraid its no more than an another example of heart over ruling the head that saw too many examples of precious troops being squandered rather than even contemplate a loss in prestige that not attempting to defend the place would bring.
Hong Kong should have been treated in exactly the same fashion as British Somaliland was the previous year - Wavell saw no strategic importance in holding the place and saw the troops as being more valuable (much to Churchill's dismay and disgust) and in the face of overwhelming numbers withdrew the Imperial forces to fight another day.
Something to note is that the 5/7 Rajput held the 'inner' line covering the retreat of the 2/14 Punjab gallantly, but the 2 Royal Scots was criticized by both Wallis and Maltby, OC Mainland Brigade and OC China Command respectively for their basically useless officers and poor discipline among the troops. By the way the 1 Middlesex was a 'static' unit that manned the MG pillboxes on the Island and fought as piecemeal companies after their pillboxes were lost. Then the Canadians were there because of Grasett, the previous OC China Command who was a Canadian. I doubt this formation would be much of a division if shifted elsewhere - it only existed due to circumstances and would be pretty much best used to defend Hong Kong.
With the exception of those formations that fought in the BEF and those fighting in Africa only the Indian Army had any real experience so its no surprise that the 2 Indian units proved to be the better units.
As for the 2 Royal Scots - it was a long term regular formation and had been in existence since before WW1 and was one of the 'Old Contemptible' units of the original BEF so it might come as a surprise to hear that it was poorly led etc - however 1 Royal Scots had been shattered near La Paradis while defending the Dunkirk perimeter in May 1940 - so I imagine that a portion of the 2nd Battalion (and I would not be surprised if this did not include the better soldiers of the 2nd leaving it with the 'Gin Drinkers') would have returned to the UK to help rebuild the first Battalion and help defend Britain against the expected German invasion.
Also if I had been a member of the 2 Royal Scots at the time my morale would be low as well!
(The rebuilt 1 Royal Scots would later see action vs the IJA in Arakan, Kohima and Mandalay)
And this is another problem - the British Army was having to replace losses suffered in France while expanding the army so men in far flung garrisons particularly the better men - would have returned during 1940/41 leaving the 2nd string behind.
While these units might not have made a very good division in Malaya or Burma they were trained personnel and trained personnel were at a premium in the far east in late 1941 and they should have been used where they would have been of the maximum use. And that is not holding Hong Kong.