After two near misses of members getting on flights where they really shouldn't have, we don't want any of you to miss your holidays, so please double check your passport dates. It's not as simple as it used to be.
As of the UK leaving the EU, when you are entering the EU you have to follow the guidelines for entering the EU as a 3rd Country National. These rules state you need to have a 10 year passport, with at least 3 months validity after the expected end of your trip.
Previously the UK used to issue passports with longer than 10 years validity by giving you credit for any unused portion of your passport. For example if you had 6 months left in your last passport when you applied for a new passport in 1 Nov 2011, you would get a passport valid from 1 Nov 2011 to 1 April 2022. However, EU guidelines (for 3rd Country Nationals) only recognize the first 10 years of the passport, so that passport now is only valid in the EU until 1 Nov 2021. This would mean your passport would only be valid for trips up to 1 August 2021.
You can find more information on the UK government website. However their recommendation of 6 months left doesn't actually make sense unless you are travelling for 3 months. The actual rule is 3 months as you can see on the Portuguese Government website.