Warning: contains spoilers for Captain Carter #1!
Marvel's same super-soldier serum flowing through her veins, and is just as capable as any man. But she doesn't have the Captain Britain name; unfortunately, someone beat her to it.
Peggy Carter became a variant of Captain America in the first episode of Marvel's What If...? television show, in which the powerful Watcher guides viewers from alternate reality to alternate reality. Carter's decision to stay on the ground floor of the test chamber kicks off a divergent timeline; Steve Rogers is shot before the serum is injected, and Carter volunteers to become the test subject in his absence. She acquires her classic vibranium shield and a UK version of Steve Roger's classic red, white and blue suit. Captain Carter makes her debut in comics in Captain Carter #1, though her origin is quite different from her MCU counterpart.
Unlike the Disney+ show, Carter isn't thrown through a portal and transporter to the present day. Much like Captain America before her, she's frozen in the Arctic for decades until modern-day scientists find and revive her. Carter takes some time acclimating to the modern world, and is even asked to become a British hero again (because most heroes, according to the British PM, are American). She still isn't given the title of Captain Britain, however; that honor belongs to one Brian Braddock.
As the English version of Captain America, Braddock received his powers from a supernatural source: Merlyn and her daughter Roma. Braddock becomes the UK's defender in much the same fashion as Captain America. Unfortunately, his books never sold quite as well as his Yankee inspiration. Multiple individuals have become Captain Britain since, but the name is well-known enough to be reserved for Braddock should he ever appear in Captain Carter comics - or the MCU.
It is regrettable that Captain Carter appeared before Captain Britain, yet the two characters were not merged together. Perhaps the two will meet and work together in the future. If both Sam Wilson and Steve Rogers can be Captain America, surely Peggy Carter and Brian Braddock can share the name of Captain Britain.
