Here are some things to consider:
* She was born and raised in England: Emma Watson was born and raised in Paris, France, but her family is British and she moved back to England when she was five years old. She attended British schools and grew up immersed in British culture.
* She's a professional actress: She has been trained in acting, and accents are part of an actor's toolbox. It's very possible that she has honed her British accent to perfection through years of practice and coaching.
* It's natural for actors to adopt different accents: Actors often have to portray characters from different backgrounds and nationalities, and this requires them to use a variety of accents.
* Subjectivity of accent perception: What one person considers "fake" another may consider "accurate" or "natural." There's a lot of subjectivity involved in judging accents.
Therefore, it's more accurate to say that Emma Watson's British accent is a performance rather than "fake". She has likely worked hard to develop and refine her accent, and it's quite plausible that it sounds authentic to many people.
Ultimately, whether you find her accent convincing or not is a matter of personal opinion.