"Nice to be working in England. Renee and Colin are an absolute joy to work with! A good couple of days rehearsing at Pinewood Studios. I really like it here!" he captioned the snapshot.
Word of Dempsey's casting broke this month, but the actor's character is being kept under wraps. The movie will follow Bridget (Zellweger) and her fiancé, Mark (Firth), as she prepares to become a first-time mother in her 40s.
Bridget Jones's Diary is based on the Helen Fielding novel of the same name, and earned over $281 million at the box office following its release in 2001. The 2004 sequel, Bridget Jones: The Edge of Reason, was nearly as successful, raking in over $262 million.
Dempsey is best known for portraying Dr. Derek Shepherd on ABC medical drama Grey's Anatomy, but was killed off on the season 11 finale in May. The actor has also appeared in such films as Made of Honor, Valentine's Day and Transformers: Dark Side of the Moon.