The Most Basic of Outlines

investigating the first conspiracy will be book one (based on the classic three-book novel structure)
investigating the second (foreign) conspiracy will be the second book
and the third book will be the invasion

It's set in 1874
Mark Antony's Parthian campaigns are successful.Cleopatra and Mark Antony defeat Octavian at Actium.
In the capital of Egypt, Alexandria, we have the High Priestess of the Universal Alexandrian Church of Jesus the Uncrucified. Her name is Cleopatra XXXIV, and she is Empress of Egypt. The Cleopatra-Antony dynasty has ruled for most of its existence, though there was some trouble with a "Mohamet". However, centuries of massive inbreeding have taken their toll, and they're kinda unstable at the top, so the power is behind the throne.
The Alexandrian sphere includes Scythia, Bactria, Parthia, India, Egypt (Syria, Palestine, Arabia, Egypt, Algeria and Tunisia), and Hellas (modern Romania, Balkans, Greece, Turkey).
The Britannic Empire is the greatest of the nations that stand against Alexandrian hegemony, along with Normandy, Gallica, Germanica, Iberia, and Nordland.
The powers-that-be in Alexandria want to obliterate Britannia, so they start to conspire to invade. They decide to weaken Britannia with a fake domestic conspiracy to kill Queen Victoria.
This is where our adventure begins.