In the 1300s? They were so rare that they were simply no laws about whether they could hold power. Occasionally one would turn up, but they held, legally, no different rights from anyone else.

Actually, unlike America, England has never, ever, had laws about race and power. Of course, a black man in 1700 could not hold power in England... because he wouldn't be descended from an upper-class family! (But it would be possible for a mixed-race person.) Blacks were just as disenfranchised as any other white person from a poor family.

The earliest black people in England that we have pictures of are black servants of Catherine of Aragon. She had several black servants, one of whom is depicted in a tapestry from the time: "John Blanke, the blacke trumpeter" was paid 8d a day by Henry VIII. He would have had exactly the same rights as any other servant... meaning he was entirely inelligible to hold power, unless given a lordship, just like white servants. (This is not to say that the English at the time treated white and black servants the same: black servants were treated very much as exotic decorations)

Now, saying that, we do have evidence of black people before 1500. In Gloucestershire, a sub-Saharan woman's skeleton was found, that has been dated to between about 900-1000 AD. But we don't have any information about who she was or how she got to Britain.

There is also, of course, evidence of black people in Roman Britain, but they interbred with the locals enough to be unnoticeable beyond DNA.