The Wantsum Channel no longer exists in our timeline, but for Britannia, it was a route for the most important trade from Londinium to Gaul and the rest of the Empire. It’s bustling port in was used to having military triremes move between it, but in July 398, a massive fleet of 700 men docked at Ruculbium, and began unloading.
Rufinus in the Eastern half of the slowly declining Empire would not hear about the charismatic general retreat from Roma until he had arrived. Having already been looking for ways to remove this thorn in his side from office, Julius Caesar-style, he would initially be relieved when an adviser told him he was out of the picture. This would turn to rage, when his adviser mentioned the departure of Honorius, boy Emperor of the West, he immediately ordered for talks with those loyal to him in the Western senate to arrange for his arrest.
The general had been new to court politics, but Alaric’s raids on Thracia and his escape in Macedonia had already told him that Rufinus was not to be trusted. The province on the outskirts of the Empire provided Stilicho with a convenient escape from court politics. Londinium was in awe when his cart with him and the Emperor arrived and walked towards the governor’s district, escorted by Catuvellauni spearmen marching alongside Roman Legion. Since his sudden arrival was unannounced the governor of Britannia Superior was shocked, especially as Stilicho waved two British soldiers towards him and he was dragged out of his office.
Honorius first decree issued from Britannia, with consent of his guardian, was to close all docks immediately. Trade stopped, and political unrest began to spark. But from military experience, Honorius knew that unrest was not as deadly as a dagger to the back.
The governor of Africa, Heraclianus (Stilicho’s assassination and Honorius’s usurper in our universe), was placed in Rome by Rufinus as the successor to the boy emperor. Seizing the opportunity to invade early, Alaric laid siege to Mediolanum. The capital would fall without a competent general in charge, and although Heraclianus would flee the city in time, the Roman armies he controlled would be utterly crushed on Easter Sunday in 402 at the Battle of Pollentia. Forced to give into Visigoth demands, the du-jure Emperor (in the eyes of the East) handed over the title of Magister Militum to the Dacian King, alongside rations of food. The Western Roman Empire would remain in control of Alaric for the time being, a situation Rufinus did not fully object to.
Meanwhile, in Britannia, Stilicho would not see much conflict with Romans from Europe. A single province on the outside of the Empire, however economically powerful, would not be worth an invasion. For all the Londinium government would be concerned, they were the true Roman Empire. The governor of Britannia Inferior, Claudius Julius, would not object, partly out of respect for who he believed was the legitimate emperor of Rome, partly out of fear of meeting the same fate as the governor of the Superior.
The Roman Empire, for all living on the island, now consisted of two provinces. One, Britannia Superior, a bustling province with loyal tribal auxiliaries and legionnaires taken from Europe. The other, Britannia Inferior, a rural land of tribes opposed to the Empires occupation of their land, supressed brutally by new government of Londinium.