Yes, there were several American actors in the Harry Potter films who performed with a British accent. One notable example is the actor, Josh Herdman, who portrayed Gregory Goyle in all eight films. Herdman is British, but there were also several American actors who played British characters, such as:

1. Eddie Redmayne - who played Newt Scamander in "Fantastic Beasts and Where to Find Them" and its sequels - he is actually British but he has performed in American films with an American accent.

2. Alison Sudol - who played Queenie Goldstein in "Fantastic Beasts and Where to Find Them" and its sequels - is an American actress who performed with a British accent.

3. Samantha Morton - who played Mary Lou Barebone in "Fantastic Beasts and Where to Find Them" - is also an American actress who performed with a British accent.

4. Jon Voight - who played Henry Shaw Sr. in "Fantastic Beasts and Where to Find Them" - is an American actor who performed with a British accent.

5. Ezra Miller - who played Credence Barebone in "Fantastic Beasts and Where to Find Them" and its sequels - is an American actor who performed with a British accent.

These actors are just a few examples of the many American actors who have portrayed British characters in the Harry Potter and Fantastic Beasts films, demonstrating their ability to master the British accent.
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