Assistant-type characters like:
- Alfred in the Batman movies
- Zazu in The Lion King series
- C-3PO in the Star Wars series
- Curator in Ready Player One
- Jarvis in Iron Man
often have British accents. Why is that?
There is no real official reason but it's mainly because of the (old) British culture.
Namely, butlers were/are servants in large households who tended to their employer’s wishes. Despite being in a servant position, they were often respected for their skill and loyalty (even today there are specialized butler schools, often located in England) which often made a good butler a status symbol.
After the colonization of the Americas, butlers were also part of the migration wave, taking care of the families in the US like they did in England. So the British accent comes from the fact that most of them were British (and also partially because many believe the British accent to be more cultured/snooty compared to other accents).
It's more than just a British accent. It's a specific kind of British accent officially known as "Received Pronunciation." It conveys a level of refinement and social prestige. When an assistant talks this way in an American movie, the implication is that they are more than just an assistant. Unsaid is that they are educated and connected, and certainly know more about whatever is going on than the boss does.
I should add that Michael Caine is one exception. He speaks with a Cockney accent, which is pretty much the opposite of RP.