Copied from other thread
Even without a real reason taking all the Britainic isle is [AH] doable.
When Emperor Claudius launched his campain in 43 AD he faced a Celtic federation that was a lot more organized, than Caeser had when He invaded 100years before. As Claudius' campaing worked it's way North over the next 30 years the Celt's and the Picts United and adapted till in 122 AD emporer Hadian ordered the legions to retreat from the firth of Forth back to the Solway Firth and build a wall.
But WI the first invasion had suceeded.
Caeser invaded in 55 BC to prevent the Britians from supporting the Gauls, Caeser was Fighting.
Caeser invaded with 2 legions, But storms prevented the landing of the cavalry , and damaged the ships He had come in.
When the Britions attacked His camp, Caeser withdrew.
Caeser returned the following year
In July of 54 BC, Caesar made the trip with 800 ships transporting five legions, 2,000 cavalry troops and their horses plus a baggage train. They sailed from Boulogne at night on July 6. He landed unopposed in an area between Sandwich and Deal. Upon seeing the large size of the Roman force, the natives moved inland with Caesar and and his troops in pursuit, marching a further 12 miles inland. At daybreak on the 8th of July, 54 BC, the Romans encountered British forces at a ford on the Stour (later the town of Canterbury). The Romans easily scatterd the Britons, who retreated to a hill fort or stronghold (oppidum), which from Caesar's description, is probably the hill fort at Bigbury.
Things didn't go all the Roman's way. For the second time in as many years, storms struck the coast, forcing Caesar's troops to beach the entire fleet and build a land fort for the ships in just ten days. While this activity held the Romans attention, the British obtained a new commander, Cassivellaunus. Cassivellaunus used guerrilla warfare tactics against his Roman visitors. However, he was not well-liked by a number of local tribes, and as a result, the Trinovantes, Cassi, Ancalites, Cenimagni, Segontiaci, and Bibroci tribes switched their allegiances to the Romans.
Cassivellaunus' slip-up occurred when he ordered his troops to attack the Romans' Deal beach camp. The failure of the attack led to Cassivellaunus' surrender, but apparently its terms were less important than new problems Caesar faced in Gaul. Caesar would leave Britain in September of 54 AD. Caesar never again came to Britain. For the next few years, he was at war with Pompey, and then he was assassinated, just when he was on the verge of becoming emperor. The next Roman invasion of Britain - and the start of over four centuries of ocupation - would not take place for another 97 years until AD 43 under the command of Claudius.
So lets use the good old AH Handwavium, and change the 55BC storm.
Caeser moves inland with his 2 legions and their calvary support.
By the time winter quarters come around, Ceaser controls quite a bit of ground,
Both by his Legions, but more important by Holding Hostages from the tribes He has allied with.
When he returns in the spring with three more legions, He finds the two Legions have expanded and reinforced their Camps, and Rome is in control of most of the Canterbury region.
By fall when Caeser returns to Gaul for the winter, Rome and Allies are in control of the entire SE England.
Ceaser would get involved in a war with Pompey in 53 AD and never return to England,
but his Seconds in Command would push North and West, Often bypassing pockets of resistance,
By 10 AD Rome reachs the north Sea tip of the Island [this is ten years longer than OTL's 43 AD~77AD].
And turns to begin cementing Its control over the Island.
In the South Roman Builders have began to widen, Repair, and reseal the Druid roads that criss cross the Island.
Meanwhile British goods are being feed into the Roman Trading Network.
While there would be a few revolts, as the British learn what it means to be a Ally of Imperial Rome,
The reality of Roman control remains secure.
This does two things, This establishes Roman controll some 50 years earlier than OTL, and extends it to the entire island.
The British islands wouldn't have an army to keep dragging away to fight over the imperial throne in the rest of the Roman empire. Instead, it has a navy to defend against pirates. A navy of no use once the troops that can be recruited in Britain are sent over the channel into Europe.
If the main Military force in Britian is Naval, Then you will have Roman Marines instead of Legionares, keeping the Peace.
This means less chance of conquering the Island of Erie.
You may also have a much more Maritime tradition in Britian. Which means a lot more contact with Erie.
So Probally a Vassal relationship then.