Passports introduced in 2015-2016 contains 9 digits printed on page 1. These are entered on the bio data page (page 2) in the same style as the personal details.
The serial number is perforated from page 1 through to the rear cover of the passport.
The thin film patch contains a unique serial number which consists of 3 alpha characters and 4 numeric followed by a check symbol. This number is located beneath the holder’s image should not be confused with the 9 digit passport number.
See GOV.uk's publication: Basic Passport Checks to learn more.
HM Passport Office is the sole issuer of UK passports and responsible for civil registration services through the General Register Office. Prior to 2013, HM Passport Office was known as the Identity and Passport Service.
Here's how to renew your adult passport.
Visit GOV.uk - Passports to know more.