In 2004, she joined the cast of the crime drama, Silent Witness. As of 2018, she is still in the show and has now played the role of Nikki Alexander for fourteen years. Also in 2004, she played Lady Margaret in Part 2 of Gunpowder, Treason and Plot, the mini-series about James I (James VI in Scotland) and the Gunpowder Plot. In 2005, Fox appeared in the BBC miniseries The Virgin Queen as Amy Dudley, the first wife of Robert Dudley, played by Tom Hardy. In 2008 she played Sister Jean in Baillie Walsh's Flashbacks of a Fool, which also featured Daniel Craig. She also starred in Things To Do Before You're 30 with Billie Piper, who would later marry her first cousin Laurence Fox, although in 2016 they divorced. She was cast as Lynne Frederick in the 2004 film The Life and Death of Peter Sellers, which starred Geoffrey Rush in the lead role. A whole section of the film focusing on the Frederick/Sellers relationship was removed in the final edit, although she can be seen briefly in a background shot towards the end of the film. The deleted scenes with Fox can be found among the special features on the DVD release of the film.