The former EastEnder will play the latest new recruit to the show - DCI Sacha Millard - who is in charge of the team of retired police officers looking into unsolved crimes.
Tamzin, who made a guest appearance on the show in another role in the last series, will appear in the final two episodes of the new series.
She said: "I'm delighted to be joining the cast at such an exciting time - I've been a huge fan of the show for years - and can't wait to be working with the amazing cast and crew."
Another of the BBC One show's stars, Alun Armstrong, will bow out in the fourth episode while Amanda will leave in the eighth programme.
James Bolam went last year, leaving just Dennis Waterman from the initial line-up, with Denis Lawson and Old Fools And Horses star Nicholas Lyndhurst joining the show.
Executive Producer Richard Burrell said: "Having another popular and talented actress to lead the team is fantastic for us and the viewing public.
"The combination of Tamzin, Dennis Waterman, Denis Lawson and Nicholas Lyndhurst ensures that the audience will continue to enjoy the antics of UCOS as they enter their second decade of pursuing justice for the victims of unsolved crimes in their own inimitable way."