If he went back a couple of decades and replaced "WWII" with "WWI" then his whole schpeel would have an actual leg to stand on. It's fairly agreed upon that the Empire began its slow death spiral after 4 years of Trench Warfare and poor planning sapped them of their manpower, stored wealth, and goodwill within the Commonwealth. They then spent the 20s relying on the Americans to help keep them afloat with the Washington Naval Conference more or less confirming their demotion to "Little Brother" status within the Anglo-Alliance they had going on with the United States. Yanks literally dictating Royal Naval policy.
It also didn't help that Winston Churchill fucked up the proposed return to the Gold Standard in 1925, fixing it at the pre-war rate instead of the actual current value because his pride refused to accept the reality the pound had begun depreciating. So he more or less guaranteed it would lose out to the Dollar. All of this was happening before Hitler even became chancellor and demanded land. Winning that conflict with Nazi Germany may have been the only thing keeping the "Britannica" illusion alive for another decade and a half, however, that's another debatable topic.
They basically became Spain after the War of the Spanish Succession, which itself is sometimes considered "WW0".
