During the period of Roman control over Britain there wasn't an native Britain king who ruled over ALL over Roman Britain certainly.

So early in the provinces history when the Romans landed the native Britons were divided into numerous relatively small tribal kingdoms. Many of these were crushed entirely but for some time those who complied with Roman rule were allowed to hold onto some status as clients of Rome, administrating at least some of their old territories. This however was eroded down over time and these territories were integrated, though in the least Romanised parts of the province local native leaders remained important in local governance.

As a whole early on the province had a governor, as an imperial rather than senatorial province this was appointed by the emperor, drawn from the eques social rank of Romans. Given they were also the commander of the troops stationed in Britannia and it was a frontier province this would often be a man of military experience and rank.

Over time though the province as many others were divided up into additional province. In the late 2nd century Britannia was divided into two, superior and inferior Britannia, this made administration a little easier and meant that individual governors had less power and less troops under their control making it harder for them to made a bid to declare themselves emperor. A little later in 212 the native Britons, along with every other freedman in the empire, were given Roman citizenship which in areas of Romanisation further blurred the line between Briton and Italian Romans.

After the Crisis of the Third Century Roman Britain like many other provinces was divided again, initially into three provinces with a fourth appearing shortly after and a fifth again at some later point.
These collectively were partly overseen by a Vicarius. Military power meanwhile was somewhat separated where the governor gave direct command of the provinces troops to a separate position of a dux though the dux was still accountable to the governors.

Later as Roman Britain collapsed, the legions largely withdrawing from the province to leave it to its fate there are a number of individuals sometimes described as king of the Britons such as Vortigern and Ambrosius Aurelianus. Actually held control of all Sub-Roman Britain is unknown.