It begins in the future, when a group of stereotypical Neo-Nazis gain control of a time machine. With the vast powers of time and space at their disposal, they decide to help Germany win WWII ( of course) by crippling Britain.
On November 12 1940, in a timeline already altered by minor time-travel butterflies, a nuclear bomb is delivered to Westminster via time portal. Westminster is leveled by the blast and the time portal is destabilized (if those wacky neo-nazis had read the manual they'd know that mixing nukes and time machines its a bad idea, drowing to engulf the entire city of London, which is conveniently moved intact to the year 60AD (where magic is oddly real, though extremely rare). (This also happens to erase the very concept of National Socialism from history, effectively cutting the Neo-Nazi agents in London from their future.)
The result is a setting where a bunch of Londoners under the leadership of Liz II (who is only 14, but is the Queen none the less, thanks to the nuke) try to avoid starving to death and keep their infrastructure from collapsing while the Romans are marching through Britain beating down every Celtic tribe in their way and a few with Neo-Nazis with limited atomopunk tech try to manipulate theis to their advantage.
All in all, its a good exceuse to have Queen Elizabeth wield Excalibur against uranium-powered cyborg Legionaries alongside Boudica, who has traded in her chariot for a tank that transforms into giant power armor.