Conflating the actions of a government with it's people. Even in a democracy that leads to dehumanizing. Love the people, respect the people, win over the people, topple the government.
Edit: you gotta remember that they are being told that anyone against them, or perceived to be, is wrong and bad and these are the reasons. Do you win that over with further hate? Do you win that over with unification and respect?
If anything, German composers seem to have became more popular in Britain at that time. I can only hazard a guess, but it may have been symbolic -- a rejection of the Nazi claims to all of German culture and German exceptionalism.