In parliament, Cook joined the left-wing Tribune Group of the Parliamentary Labour Party and frequently opposed the policies of the Wilson and Callaghan governments. He was an early supporter of constitutional and electoral reform (although he opposed devolution in the 1979 referendum, eventually coming out in favour on election night in 1983) and of efforts to increase the number of female MPs. He also supported unilateral nuclear disarmament and the abandoning of the Labour Party's euroscepticism of the 1970s and 1980s. During his early years in parliament, Cook championed several liberalising social measures, to mixed effect. He repeatedly (and unsuccessfully) introduced a private member's bill on divorce reform in Scotland, but succeeded in July 1980—and after three years' trying—with an amendment to bring the Scottish law on homosexuality into line with that in England.