That leak occurred on July 22, 2016. The FBI opened its counterintelligence investigation on July 31, 2016. Peter Strzok, the FBIs deputy chief of counterintelligence at the time, oversaw the investigation.
Strzok texts suggest he was in London on August 3, 2016:
Page attended the symposium just four days after his July 2016 Moscow trip. Page met with Halper during the London visit. Pages Moscow trip would figure prominently in the Steele Dossier
Halper also requested and attained a meeting with another, unknown, Trump Campaign member on September 11, 2016 two days before the Papadopoulos meeting.
Halper offered to help the campaign but did not mention the Papadopoulos meeting to the second staffer nor the staffer meeting to Papadopoulos.
Curiously, despite the offers of campaign help, on November 3, 2016, Halper publicly stated that Clinton would be the best option for US-UK relations.