The biggest threat to India in the 20th Century is not Russia, but Japan.
In 1905, while Japan had just thrashed the Russian fleet, it was still having trouble taking Korea. The Russians cried uncle because their economy was going to trash too, and Revolution had almost come; had they held on longer, the Japanese economy would have come crashing down, bringing down the Japanese war effort with it.
And even if Japan were seen as that strong, there was still the massive absorbent mass of China in the way. Hindsight, as they say, is always 20/20, and everyone thought that Japan would have its hands full handling China (which, to be honest, was correct even at the height of WW2, as the Japanese were unable to fully subjugate the coast and were unable to advance too deeply into China's interior. Their advances into Indochina were possible because they took advantage of the Vichy government and a severe lack of preparations in that part of the world. Furthermore the Japanese in WW1 were gentlemen, while they became outright savage in WW2. That would have required a psychic to realize how aggressive the Japanese would become by 1937, especially if you're looking at them as the country that just came out of the Meiji Restoration.
When France capitulated Britain should have taken the initiative and invaded FIC, in the same manner that Britain invaded Madagascar and attacked the French fleet in the Mediterranean and North Africa. That’s the sort of prudent yet aggressive action the empire needed, take FIC before Japan can, and tell Japan that Britain will remember if Japan makes any move while Britain is busy in Europe.
Except Europe and Africa were literally on fire in WW2, as the Axis had decided to invade the Balkans, the BEF had to dump most of its equipment at Dunkirk to make room on the boats, France had fallen and with it North Africa, leaving Gibraltar and Egypt woefully exposed to a hostile Mediterranean. Oh, and there was a bewildered and terrified British public fearing that the Germans would just hop across the Channel and force a jackboot on Britain's throat. There were other priorities at the time, as Madagascar was invaded in 1942, when the tide was turning, and the attack at Mers-El-Kebir was less an act of decisiveness and more a terrified British navy trying to neutralize a major threat. And the latter had repercussions long after the war, with bad blood between the British who saw it as necessary, and the French who saw it as dishonorable slaughter.
Most of the Empire's meager land forces were tied up in Europe and Africa, so they had little to spare in the Far East. And then the Japanese massacred the
HMS Repulse and
Prince of Wales with aircraft, effectively showing that the old Royal Navy was sorely obsolete in a naval battle there. To show strength there would have required pulling it away from the far more critical zones in Egypt and the Mediterranean.
Doing all that before WW1 would be massively out of context, make Britain the aggressor for actions that clearly have no cause (unless the British can explain they've suddenly gotten the ability to see the future clearly) and turn the world against them.
Britain may have been the greatest power of the age, but by 1900 things had vastly changed. And even at its height, Britain was less about aggressive power and more about maintaining a careful balance and preserving trade.
Instead Britain, especially in the post-ww1 to Churchill era was governed by pussies, gents who dithered, prevaricated and procrastinated on what to do. That’s not the men that can build or keep an empire, and Japan had their measure.
The people who elected such 'pussies' were the people who had experienced the horrors and perils of World War 1. If they've been "castrated" as you put it, it's because they walked into WW1 expecting another glorious war, and got the Somme, Passchendaele, and all sorts of horrible battles, which scarred the British and French so badly they never wanted another major war like that ever again. Bullish, confident rhetoric had been common before WW1, but after that people got a lot more anxious about talking big because they themselves found nothing but death and horror in the trenches. The German outrage at the Treaty of Versailles outweighed their horror, and even then, they wanted to believe Hitler when he said war will be swift and short and with minimal (German) casualties. Hell, the blitzkrieg was designed almost entirely around breaking up enemy strength while taking the least amount of casualties in the process.
Excellent points, you beat me to them - and got a few I hadn't realized.