After Hitler's power over the youth is revealed to be genetic manipulation of anyone in England born after World War II, John behaves as if this does not apply to him. Yet, the actor Nicholas Young was born in 1949 (four years after the war) and the series has repeatedly hinted that John is only in his early 20s. Thus, John should be affected by Hitler's charisma along with the rest of Britain's youth.
When Hitler mind controls the British army soldiers, he states that they are the first British men under his command. Historically, during the Second World War, hundreds of British fascists joined various branches of the regular German military while a full unit of the Waffen-SS (the British Free Corps) was formed nearly entirely from British prisoners of war.