1. Hugh Parkfield = Mandy Patinkin
In Lisa's Wedding, Mandy Patinkin (who isn't British) played the very British future almost-husband of Lisa.
2. Akira = George Takei
In addition to voicing Akira in One Fish, Two Fish, Blowfish, Blue Fish, George Takei also voiced Wink in Thirty Minutes Over Tokyo and the Chinese waiter in The Old Man and the Key.
3. Christopher Walken = Jay Mohr
When The Simpsons includes a celebrity appearance, they almost always are able to get the actual celebrity to do the voice - and in the rare case they can't, they have one of their regular voice actors step in for the impersonation. Yet they brought in comedian Jay Mohr to voice Christopher Walken in Insane Clown Poppy after Christopher Walken cancelled his own recording.
4. Allison Taylor = Winona Ryder
The character was named after writer Mike Scully's two daughters, Allison and Taylor.
5. Space Coyote = Johnny Cash
The top choices for the voice of the Space Coyote in El Viaje Misterioso de Nuestro Jomer (The Mysterious Voyage of Homer) came down to Johnny Cash and Bob Dylan. Since Bob Dylan had previously rejected opportunities to appear on the show, the role was offered to Johnny Cash, who accepted.