A post-dated passport only covers a name you take through your marriage, such as your spouse’s surname or a double-barrel of both. For anything else, HM Passport Office needs a change of name document such as a deed poll, and a deed poll takes effect the day you sign it, so there’s no such thing as a post-dated one.
That’s why couples who want a blended surname, a new first name or a maiden name kept as a middle name sign a deed poll and apply for an ordinary passport. You’ll also need evidence that you use your new name, such as a payslip, a letter from your council or a driving licence issued after your deed poll, so leave plenty of time. GOV.UK: other name changes on a passport
Taking your partner’s surname before the wedding works the same way. Our guide to taking your partner’s surname without marrying explains how.