In addition to passport being expired but also various requirements re how long is left on the passport by the date of the return journey, time period from date of issue, etc Not going to attempt to explain but look at the GOV.U.K website for full details. I’ve just renewed mine as the expiry date was too close to our return from Europe. A bit annoying as not getting the ten years validity. On the plus side with the digital photo access system used by Passport Office DVLA etc the renewal was easy and new passport received in less than a week from application.