On page 9, collaboration with partners is discussed.

p. 10 "Our Modern Slavery Steering Group, comprised of representatives from Procurement, Ethics and Compliance, Responsible Business, Legal, HR, Internal Audit and appropriate commercial teams, continued to convene to review and oversee progression of the modern slavery strategy, and set the approach for compliance across the business. The programme is overseen by Centrica’s Safety, Environment and Sustainability Committee, comprised of independent non-executive directors, with updates provided to the Centrica Board. "

This is a Modern Slavery statement in compliance with the UK Act and so the company is not (legally) required to consult it's subsidiaries nor is it included in the reporting guidance for companies. 

p. 11 shows it's a group statement "For and on behalf of Centrica plc and its subsidiaries."

and the quotes shared above (p. 10) shows there was consultations across the business, on their approach to modern slavery.