I know my facts. Hope this helps:
Before September 10, 2018:
If you renewed your passport before the old one expired, the unexpired time (up to 9 months) would be added to the new passport's validity.
After September 10, 2018:
This practice ended, and passports issued after this date are valid for exactly 10 years.
Why the change?
This change aligned the UK with other countries, which generally issue passports with a maximum validity of 10 years.
Impact on travel to the EU:
Since the UK left the EU, British passports issued more than 10 years before travel to EU countries (except Ireland, Iceland, Norway, Lichtenstein and Switzerland) are no longer valid.