Question: What British Prime Minister did Wilberforce serve under who became party leader in 1975?

Context paragraphs:
Beaconsfield by-election, 1982: The Beaconsfield by-election, 1982 was a parliamentary by-election held on 27 May 1982 for the British House of Commons constituency of Beaconsfield in Buckinghamshire.  It is notable for being the only election Tony Blair lost in his 25-year political career.  He was elected to parliament for Sedgefield, County Durham in the following year's general election, became party leader in 1994 and went on to become prime minister with three consecutive general election wins.

United for Latvia: United for Latvia (Latvian: "Vienoti Latvijai" ) is a center-right populist political party in Latvia.  It was founded in 2011 in Rēzekne and in 2013 Ainārs Šlesers became party chairman.  Šlesers has been dubbed one of the three "oligarchs" prominent in Latvian politics, and United for Latvia marks his return to electoral politics.  He was previously leader of Latvia's First Party/Latvian Way and has served in several governments of Latvia including as Deputy Prime Minister in the government of Aigars Kalvītis and Minister of Transport in the second government of Ivars Godmanis, both of whom are party members.  Other formerly prominent Latvian politicians who are now party members include Jānis Jurkāns and Jānis Straume.  The party is yet to win seats in the Saeima or the European Parliament.  Šlesers and several other party members have been in the past among the Latvian politicians most willing to cooperate with Harmony and other Russian parties in Latvia.

30th General Assembly of Prince Edward Island: The 30th General Assembly of Prince Edward Island was in session from March 29, 1887, to January 7, 1890.  The majority party was the Conservative Party led by William Wilfred Sullivan.  After Sullivan resigned to serve in the province's Supreme Court, Neil McLeod became party leader and premier.

Edward Heath: Sir Edward Richard George Heath, (9 July 1916 – 17 July 2005), better known as Ted Heath, was a British politician who served as Prime Minister of the United Kingdom from 1970 to 1974 and Leader of the Conservative Party from 1965 to 1975.  He was a strong supporter of the European Communities (EC), and after winning the decisive vote in the House of Commons by 336 to 244, he led the negotiations that culminated in Britain's entry into the EC on 1 January 1973.  It was, says biographer John Campbell, "Heath's finest hour".  Although he planned to be an innovator as Prime Minister, his government foundered on economic difficulties, including high inflation and major strikes.  He became an embittered opponent of Margaret Thatcher, who supplanted him as party leader in 1975.

Wilberforce (cat): Wilberforce was a cat who lived at 10 Downing Street between 1973 and 1986 and served under four British Prime Ministers: Edward Heath, Harold Wilson, Jim Callaghan and Margaret Thatcher.  His chief function was to catch mice, in which role he was the successor to Peta.  In life he had been referred to as "the best mouser in Britain", as befitted his role.

Margaret Thatcher: Margaret Hilda Thatcher, Baroness Thatcher, {'1': ", '2': ", '3': ", '4': "} (née Roberts ; 13 October 1925 – 8 April 2013) was a British stateswoman who was Prime Minister of the United Kingdom from 1979 to 1990 and Leader of the Conservative Party from 1975 to 1990.  She was the longest-serving British prime minister of the 20th century and the first woman to have been appointed.  A Soviet journalist dubbed her the "Iron Lady", a nickname that became associated with her uncompromising politics and leadership style.  As Prime Minister, she implemented policies that have come to be known as Thatcherism.

23rd Parliament of Ontario: The 23rd Legislative Assembly of Ontario was in session from June 2, 1948, until October 6, 1951, just prior to the 1951 general election.  The majority party was the Ontario Progressive Conservative Party, however its leader, George Drew, lost his seat in the 1948 general election and soon after resigned as party leader to enter federal politics and take the leadership of the Progressive Conservative Party of Canada.  He was replaced on October 19, 1948, by Thomas Laird Kennedy who served as premier and interim Progressive Conservative leader until Leslie Frost became party leader and succeeded Kennedy as premier on May 4, 1949.

Han Myeong-sook: Han Myeong-sook (born March 24, 1944; Korean: 한명숙 ] ) was the Prime Minister of South Korea from April 2006 to March 2007.  She is South Korea's first female prime minister (second female prime minister overall if the acting premiership of Chang Sang is included).  She was from the United New Democratic Party (UNDP) as a member of the Korean National Assembly (representative) for Ilsan-gab, and is a graduate of Ewha Womans University in Seoul with a degree in French literature.  She resigned as Prime Minister on March 7, 2007 and declared her presidential candidacy.  But she did not succeed in the nominations.  In 2008 she ran for parliament, but was not elected.  However, in January 2012 she was elected leader of the main oppositional Democratic United Party (DUP) before the April legislative elections and became a member of parliament.  But the liberals did not manage to defeat the ruling Saenuri Party and Han stepped down as party leader in April 2012.  In August 2015, Han was convicted of receiving illegal donations at the amount of 900 million KRW, and sentence to two years in prison.  She is ineligible to run for public office for ten years after her prison term.  She became the first former prime minister of the Republic of Korea to serve a prison time.

André Rouvoet: André Rouvoet (] ; born 4 January 1962) is a former Dutch politician of the ChristianUnion (CU).  First as a member of the RPF until the party merged with the Reformed Political League (GPV) to form the ChristianUnion.  He served as a Member of the House of Representatives from 17 May 1994 until 22 February 2007.  When the ChristianUnion party leader Kars Veling retired, Rouvoet unexpectedly became the new party leader and the Parliamentary leader in the House of Representatives serving from 12 November 2002 until 22 February 2007.  On that day, he became Minister of Youth and Family and Deputy Prime Minister serving from 22 February 2007 until 14 October 2010 in the Fourth Balkenende cabinet.  He returned to the House of Representatives on 17 June 2010 and again as parliamentary leader.  On 29 April 2011 he announced his retired from politics.  He was succeeded as party leader and parliamentary leader by Arie Slob.

Prime Minister's Official Spokesperson: The Prime Minister's Official Spokesperson or alternatively Prime Minister's Official Spokesman/Spokeswoman is a position in the United Kingdom's Civil Service, located in the Prime Minister's Office in 10 Downing Street and used by the British Prime Minister to convey information to the public.  The Prime Minister's Official Spokesperson usually addresses a small group of press and media correspondents, known as lobby correspondents, each morning to deliver statements on current events on behalf of the Prime Minister.

Supporting paragraphs: Wilberforce (cat) Edward Heath
Answer: Margaret Thatcher