Sheriff, who currently heads up comms for the Department for Transport, takes on the job in September and will be responsible for external communications and employee engagement for the FTSE 100 company.
Her former roles include being the UK Prime Minister’s deputy official spokesman and head of news for both the Ministry of Justice and the Department for International Development.
Sheriff will report to global corporate relations director Charlotte Lambkin, who said: "She has great experience in running and developing large teams covering a wide range of disciplines, managing complex and fast-moving issues and positioning senior individuals and organisations.
"She will be able to join together our employee engagement and external communications teams to deliver against an exciting agenda."
Sheriff added: "After a long career enjoying communications in government, I’m looking forward to bringing my skills and experience to the commercial sector."