Red Skull

The Red Skull was, according to differing accounts, either a real individual whom Captain Britain fought, (COMIC: Camelot Lost) or a fictional villain fought by Captain America. (COMIC: Moving In)

Biography
When Merlyn resurrected Captain Britain's mind, he reformed memories of the Captain's past enemies, including the Red Skull. (COMIC: A Rag, a Bone, a Hank of Hair...)

Captain Britain recalled fighting the Red Skull, referring to him as one of "the most sinister villains". (COMIC: Camelot Lost)

Other references
The Twelfth Doctor believed that Captain America would win in a fight between him and Batman because Captain America fought the Red Skull, which the Doctor believed was tougher than "some idiot in clown make-up". (COMIC: Moving In)