Major Roles:
* Hugh Grant: "Notting Hill" (1999)
* Rupert Everett: "My Best Friend's Wedding" (1997)
* Colin Firth: "The Importance of Being Earnest" (2002)
* Ralph Fiennes: "The English Patient" (1996) - Although he's British, the film itself was a co-production with multiple countries and not entirely British.
Smaller Roles or Cameos:
* Alan Rickman: "The Player" (1992) - This was more of a supporting role and while it was a well-known film, it didn't have the same impact as "Notting Hill" or "My Best Friend's Wedding."
* Benedict Cumberbatch: "The Curious Case of Benjamin Button" (2008) - This is a brief cameo appearance, and while Cumberbatch is a very famous actor now, he was less well-known at the time.
Other Notable Mentions:
* While not strictly British, Pierce Brosnan has dual Irish and British citizenship and was in the movie "The Mirror Has Two Faces" (1996).
* While not an actor, Richard Curtis (British writer/director) wrote the script for "Notting Hill," which is the most famous film Julia Roberts did with a British lead.
It's important to note that this is not an exhaustive list, and there might be other British actors who have worked with Julia Roberts in smaller roles or short films.