Prior to the Amazonian Occupation of the United Kingdom, Penelope Black was a member of the UK's Royal Navy, Technician First Class aboard the HMS Tintagel. The Tintagel was a modified Type 42 Destroyer class warship, specially designed to transport- and bench-test- a highly experimental teleportation device, known as the "Lionheart Engine." The Engine was a piece of ultra-classified kit designed to generate a frictionless field around the ship, allowing it to travel at speeds far greater than any maritime vessel could ever hope of achieving under normal circumstances. Were the Lionheart to become operational, Britain would rule the seas once again, which was good news in this age of global terrorism.
They were testing it in the Solent strait when something went seriously wrong.
Observers would later state that the Tintagel simply vanished into thin air. However, what had really happened was far more complex than that. The Lionheart, the Tintagel, and her crew had been involved in a bizarre bulk teleport of sorts, disappearing off the face of the earth for a whole week before reappearing on the Salisbury Plain, several miles inland. Every last soul aboard had been disintegrated, save Penelope, who had survived due to the previously unidentified metagene trait she carried within her DNA. Somehow, it had allowed the Lionheart technology to fuse with her body. Britain's top scientists went to work, scanning and testing extensively, finding that what they had on their hands was more than just a simple servicewoman...
But more of an unexpected experimental breakthrough, the likes of which they had never seen.
Using the same technology that went into crafting the Lionheart Engine, she was fashioned what came to be the MK I Britannia Armor, a high-powered suit that would allow her to channel and focus her abilities to their full potential. Penny had become more than just another soldier, she had become a national hero, and a poster child for the Royal Navy, not to mention an extremely valuable prototype in a new generation of biotechnological weapons. She was the Royal Navy's secret weapon, their super-soldier. As Britannia, she saw action the world over, participating in what later came to be known as "The Kahndaq Conflict" and similar debacles over the years before the Atlantis/Amazon War. During that time, she intervened on the behalf of Grifter's black ops team, all of whom had been slaughtered during a mission into Afghanistan. Due to their inherent telepathic powers, they formed a sort of impromptu bond. After healing from his wounds, he tracked her to London, in hopes of thanking her.
In the wake of the Amazonian occupation of Great Britain, Penny managed to survive their initial strike on London, finding herself separated from her armor, fighting for her life, and on the run from not just the Amazon warriors, but from the Female Furies, Princess Diana's personal squadron of highly-trained superhuman soldiers. She was eventually captured by the Amazons, but was soon freed by Grifter, who took her to one of the safehouses belonging to the Resistance cell he had formed. There, she met some of Britain's other heroes: Etrigan the Demon, Godiva, Wicked Jinny Green Teeth, Ms. Hyde, and the youngest of their number, the Canterbury Cricket.
As the Resistance, they worked to undermine the Amazons, knowing full well the atrocities they were capable of, such as the concentration camps and Purple Ray Treatment Camps. In their endeavors, they came across Daily Planet reporter Lois Lane, who had been following her photographer, Jimmy Olsen on a story in the UK before it was attacked and the Amazons took over. Aiding her in escaping the Amazon's palace, Penny and the others informed her that, prior to his death in the attack, Jimmy had been an undercover agent for Cyborg, who was attempting to gather vital intel on the Amazon Army to aid both his and the Resistance efforts to bring them down and end their war with Atlantis.
During her time with the Resistance, Penny confided in Lois about her origins and actions as a member of the Royal Navy and as Britannia. There was nothing to stop her anymore, no British Government to protect, no Official Secrets Act to observe. When the Amazons took Britain, they took everything from them, from her.
Based on the encrypted data they found in a special drive on his camera, the Resistance learned that Jimmy had uncovered the location of the secret armory that housed the MK II Britannia Armor, beneath the Palace of Westminster, London. Absconding with one of the Amazon's Zodiac Boats, they quietly made their way to the Palace to retrieve the Armor, knowing that, with Penny once again adopting the identity of Britannia, they would stand a better chance at battling the Amazons head on, and that her presence as a symbol of British patriotism would spur the interned citizens of Britain into fighting back against their oppressors. However, their plans were leaked to the Furies by Ms. Hyde, who betrayed them so that the Amazons would make good on their promise to cure her of her super-powered persona. Thankfully, they managed to turn the tide against them when said persona took control of her body and soundly thrashed the Furies, allowing Etrigan to recover from his wounds and Cricket to free himself.
With the others buying them time, Penny and Lois made for the armory beneath the Palace. From behind, the Furies' field commander, Artemis of Bana-Mighdall shot Penny in the back with an arrow, severely wounding her. Using a machine-gun, Lois held Artemis off while Penny did her best to reach the Armor, slowly bleeding to death. Upon reaching the inner-sanctum where the armor was held, Penny climbed inside and her metagene activated the armor, causing it to heal her mortal wound nigh instantaneously.
Finally back to full strength, Britannia flew into action. Her first act was to save Lois from being impaled by Artemis, surprising the Amazon from behind just before she literally tore her in half in a display of inhuman strength. Taking to the air to battle the Amazons directly, she was met halfway by Diana herself. A bloody battle ensued, with Britannia just barely able to hold her own against the Princess of Themyscira, a feat that surprised her opponent. Their battle eventually made its violent way to one of the many concentration camps that the Amazons had established to imprison the men, something that shocked and horrified Diana, bringing their fight to an abrupt halt. She claimed not to know anything about this, and was further repulsed when Penny told her about the other acts the Amazons were responsible for. Mutation camps, re-education camps, slaughter pits, etc. In a surge of conscience, Diana frees the men and leaves Britannia to go and confront her commander Penthesilea, who had perpetrated these atrocities in her name.
At the same time they had been fighting, Lois had managed to gain control of Britain's emergency broadcast network, courageously taking to the airwaves to reveal the truth about the Amazon's actions to the people, urging them and the Resistance to keep fighting. However, she was interrupted by a group of soldiers, and had to flee, but not before being rescued by Kal-El. Her message sent, Britannia and the rest of the Resistance were left to regroup and plan their next move, but not before being confronted by a small army of Amazonians and mutated soldiers intent on suppressing them once and for all.
With the recently freed men now armed, and Grifter by her side, Penny braced for what might be her final battle, leading the charge for their freedom, the lyrics for "Rule Britannia" on her lips.
"Britons never, never, never shall be slaves."
In the aftermath of the Flashpoint event, her ultimate fate is currently unknown.