To support my argument, here’s the passport information from two popular carriers:
Easyjet:
Please check your passport's expiry and issue date before you travel. If you're a UK passport holder travelling to the EU (except Ireland), or Iceland, Liechtenstein, Norway, Andorra, Monaco, San Marino, the Vatican City or Switzerland, your passport will need to meet the following criteria:
It must be valid for at least three months after the day you plan to depart from the EU or above countries*
It must be no more than 10 years' old on the date of travel to the EU or above countries.
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Jet2:
Passport Validity
As the UK is no longer part of the European Union, EU countries now require UK passports to be less than ten years old and still be valid for at least three months at the time you're due to leave their country. To check your passport is valid for travel, ask the two questions for each person on your booking (including children) – if the answer is yes, you’re ready to go!
1. On the day you arrive into the EU, will your passport’s “Date of issue” be less than 10 years old?
2. On the day you leave the EU, will your passport have at least three months remaining (based on the “Date of expiry”)?
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No doubt other carriers have similar advice. Anybody not troubling to read it can blame themselves for their lack of attention. I really have no sympathy whatsoever for anybody who is caught out by this. The fact that they were used to different arrangements is hardly the point. There can be nobody in the UK who is likely to travel abroad who is unaware that the country is no longer an EU member. They should have checked for changes.