English, Irish, French and more were involved
Culloden wasn’t just the Scottish against the redcoats. There were a number of other nationalities involved. In fact, a number of Irish and English that supported the Stuart claim to the throne backed the Jacobites and sent men to fight against the British army.
The main reason the term “British” is now used for the offensive army is to make it clear that the Redcoats were fighting on the side of the Crown. Really, it was British against British. Outlander does a good job of portraying this when Bonnie Prince Charlie makes it clear that he wants the opposition to be given the same medical care as the Jacobite soldiers. They were all his father’s subjects.
While there were multiple other nations fighting on the Jacobite side, there were also Scottish clans fighting on the side of the Crown. Some of those Scottish clans were Highland clans. A Highlander didn’t necessarily make someone a Jacobite—nor did being a Catholic! Most of the common recruits, though, were from the North East of Scotland.
The Crown was led by the Duke of Cumberland
One thing Outlander misses out is the person leading the other side of the fight. The Crown’s forces at Culloden were led by the William Augustus, Duke of Cumberland, King George II’s youngest son. He had gained the reputation as “The Butcher” due to his atrocities against Jacobite soldiers.
While he may have been named “The Butcher,” the Duke of Cumberland knew his military strategy. He helped to train his men to hold fire until the Jacobites were in effective range and ensured they were properly trained for the battle at Culloden.