Caesar had two expeditions to Britain. The first one was... kind of a bit of a fail since the Romans learned how to sail in the Mediterranean and lost ships to Channel Weather.
The next one resulted in several skirmishes (which the Roman cav won). The Romans restored Mandubracius as king of The Trinovantes. There was a battle between the Romans led by Caesar and some Britions led by Cassivellaunus, which the Romans won, leaving the latter so sit trapped in a fort. In the end Cassivellaunus, allowed Mandubracius to stay unmolested and he agreed to pay tribute to Caesar.
What if Cassivellaunus decided he couldn't face Caesar at all and just allowed them to make Mandubracius as a client king (against his interests since he previously took prime hunting ground and forests from the Trinovantes) and pay the tribute? It would mean less material losses by both sides (because they aren't fighting) and Caesar sets up his client king, but at the same time he doesn't get anything to brag about.
The next one resulted in several skirmishes (which the Roman cav won). The Romans restored Mandubracius as king of The Trinovantes. There was a battle between the Romans led by Caesar and some Britions led by Cassivellaunus, which the Romans won, leaving the latter so sit trapped in a fort. In the end Cassivellaunus, allowed Mandubracius to stay unmolested and he agreed to pay tribute to Caesar.
What if Cassivellaunus decided he couldn't face Caesar at all and just allowed them to make Mandubracius as a client king (against his interests since he previously took prime hunting ground and forests from the Trinovantes) and pay the tribute? It would mean less material losses by both sides (because they aren't fighting) and Caesar sets up his client king, but at the same time he doesn't get anything to brag about.