who were my people of the middle ages?

You need to trace your descent back from parents to grandparents to great-grandparents ("Juan, son of  Pedro, son of Phillip, son of Iago, son of Phillip, son of Carlos") back, back, back at least six hundred years to get to the names of your ancestors in the 1400s. That's a lot of research.

You also need to read histories of Spain and of the Isle of Man, to get some sense of what your ancestors did.

For instance, in the 1500s.  King Philip II of Spain was the son of a princess of Portugal and the Holy Roman Emperor Charles V (King Carlos I of Aragon and Regent for Queen Juana of Castile) The Emperor Charles was the son of Queen Juana of Castile and Archduke Philip of Flanders. The mother of the Portuguese princess was likely a sister of Queen Juana. Their parents were King Ferdinand of Aragon and Queen Isabella of Castile. Philip of Flanders father was the Holy Roman Emperor Maximilian of Austria. Max's house was the House of Habsburg.

So you see, if you were royal, your ancestors could have come from Austria, Flanders/Burgundy (or whatever Belgium and the Netherlands used to be in the 1500s), Portugal, Aragon, Castile. You would have relatives whose ancestors came from England, France, the kingdom of Navarre, the kingdom of Naples, the kingdom of Leon, Moslem kingdoms.

It's likely you are not royal, but what if one of your ancestors was Maria de Salinas, maid of honour to Catalina of Aragon (who married King Henry VIII of England)? Maria married an English baron. Her daughter Katherine married an English Duke. Katherine was born English but her parents were English and Spanish. 

So not all the English have English ancestors, and probably not all the Spanish have Castilian ancestors, or Moorish ancestors.

You need to dig into whatever papers and records you can find to trace you ancestors. It will take years and be very frustrating. My cousin has spent most of her life tracing her ancestors and found a line back to the Salem Massachusetts "witch" trials. (So of course she had to read about the 'witch' trials and the history of the New England Puritans as well as trace the genealogy. NOTE: the connection is not on our mutual grandparents' side so I can't claim it. Still it is something to know now when I read about the 'witch' trials.) Your Manx mother is British, but not "English". At least she is "English" as the people of Cornwall are "English"; but I suspect that a Cornishman would correct me vigorously, and so would your mother.  "I am Not English."

Your peoples could have been nobles or fishermen or migrants or thieves or farmers, but we cannot tell you. You have to seek it out, your parents to their parents to their parents and so on from whatever records you find.

You need to look further back than your parents. Grandparents, great-grandparents and so on.

There is no way to tell. The middle ages lasted a thousand years and ended 600 years ago, People have moved around a lot between then and now.