What the Passport Office needs
If you’re going back to the surname you were born with, HM Passport Office’s guidance treats it as reverting to your birth name. It asks for your full birth certificate, your marriage or civil partnership certificate, a signed statement that you’re reverting and evidence that you use your birth name. It doesn’t ask for a divorce document. HM Passport Office: change of name applications
If you’re choosing a name that isn’t your birth name, the Passport Office treats it as a new change of name and asks for a change of name document such as a deed poll, plus evidence that you use the name. GOV.UK: other name changes on a passport