The opera singer was one of the first to arrive at St Paul's Cathedral to pay her respects to the late Prime Minister.
Katherine was pictured outside the main entrance of the cathedral dressed wearing a black blazer and skirt and a Philip Treacy hat.
Actress Joan Collins was also among the 2,300 guests at the service, and was seen arriving with her husband Percy Gibson.
The former PM voted against Section 28's repeal in the House of Lords, which he described as "throwing a piece of red meat to her right-wing wolves".
He wrote: "Lest we forget, this nasty, brutish and short measure of the third Thatcher administration, was designed to slander homosexuality, by prohibiting state schools from discussing positively gay people and our 'pretended family relations'."
He added that Section 28 "galvanised a new generation of activists" before it was finally repealed in 2009 under Tony Blair's government, with David Cameron later apologising for the policy once he came into power.