Jessica Rose Brown Findlay (born 13 September 1989) is an English actress, most widely known for her performance as Lady Sybil Crawley in the ITV (UK) and PBS (U.S.) television period drama series" Downton Abbey", and for her role as Emelia Conan Doyle in the 2011 British comedy-drama feature film "Albatross".
Jane Wymark (born 31 October 1952) is an English actress.
 The daughter of English actor Patrick Wymark (1926–1970) and the American writer and playwright Olwen Wymark, she is best known for playing Morwenna Chynoweth Whitworth (Morwenna Carne by the close of the series) in the 1970s BBC television period drama "Poldark" (1977), and more recently as Joyce Barnaby (1997–2011) in the ITV detective series "Midsomer Murders".
 She has appeared in UK television dramas such as "The Bass Player and the Blonde", "A Touch of Frost", "Dangerfield", "Lovejoy" and "Pie in the Sky".
 She also appeared as Jill Mason in the Birmingham Rep production of "Equus".
The Chinese Detective is a British television police procedural drama series, first transmitted by the BBC between 1981 and 1982.
 The series was created by Ian Kennedy Martin, who had previously devised "The Sweeney" and "Juliet Bravo".
 The series starred British Chinese actor David Yip as Detective Sergeant John Ho, the first Chinese lead actor in a British television drama series.
 The series offered traditional police procedural storylines in a setting of occasional prejudice and distrust within the police force, and the prejudice displayed by those Ho encounters whilst doing his job.
 Like many other television detectives of the time, Ho was something of a maverick, often using unorthodox methods to solve crimes.
 The series was set in and around London's docklands, before redevelopment began in the 1980s.
Michelle Suzanne Dockery (born 15 December 1981) is an English actress and singer.
 She is most widely known for her performance as Lady Mary Crawley in the ITV (UK) and PBS (U.S.) television period drama series "Downton Abbey" (2010–15), for which she was nominated for three consecutive Emmy Awards in the category for Outstanding Lead Actress In Drama Series, and a Golden Globe Award nomination.
 She made her professional stage debut in "His Dark Materials" in 2004.
 For her role as Eliza Doolittle in the 2007 London revival of "Pygmalion", she was nominated for the Evening Standard Award.
Laura Elizabeth Carmichael (born 16 July 1986) is an English actress, most widely known for her performance as Lady Edith Crawley in the ITV (UK) and PBS (U.S.) television period drama series" Downton Abbey".
 Her other television work includes "Marcella" (2016).
, and the feature film "A United Kingdom" (2016).
Land Girls is a British television period drama series, first broadcast on BBC One on 7 September 2009.
 "Land Girls" was created by Roland Moore and commissioned by the BBC to commemorate the 70th anniversary of the outbreak of the Second World War.
 The programme was BBC Daytime's first commission of a period drama.
 "Land Girls" was filmed in and around the city of Birmingham.
 The first series features Summer Strallen, Christine Bottomley, Jo Woodcock and Becci Gemmell as four different girls doing their bit for Britain in the Women's Land Army during the war.
Father Brown is a British television period drama which began airing on BBC One on 14 January 2013.
 It features Mark Williams as the eponymous crime-solving Roman Catholic priest.
 The series is loosely based on short stories by G. K. Chesterton of the same name.
Cara Louise Theobold (born 8 January 1990) is an English actress who trained at the Guildhall School of Music and Drama.
 Her first acting role was Ivy Stuart in the third and fourth series of television period drama "Downton Abbey" and is also known for voice acting as Tracer in the video game "Overwatch".
Joanna Horton is an English actress, notable for her stage and television work, including an episode of "Robin Hood" (series 1 episode 4), "Father Brown", "Spooks" and "Foyle's War".
 During summer 2013 she is appearing with the Royal Shakespeare Company as Celia in "As You Like It" and Helena in "All's Well That Ends Well".
Kelly Adams (born 16 October 1979) is an English actress, best known for her roles as Mickie Hendrie in the BBC One medical drama series "Holby City" (2004–2006) and "Casualty" (2005), as Emma Kennedy in the BBC One drama series "Hustle" (2009–2012), Padre Mary Greenstock in the BBC Three comedy drama series "Bluestone 42" (2013–2014) and Nancy Webb in the ITV period drama series "Mr Selfridge" (2015).

Question: Who is the lead actor in the British television period drama series that also featured English actress Joanna Horton?
Answer: Mark Williams