In October 2010 the British redisigned their passport to improve security. Some of the new security features include moving the chip storing the holder's details from the outside to the inside and including holograms which protect the holder's details. One of these important details is the passport number which can be found in three places in the passport. This number can only be obtained by looking in the passport.
Open the passport from the front and look at the very bottom of the first page on your right. The passport number can be found here in bold numbers.
Look at the very top of the first page and you will see the passport number has been perforated across the top of the page. This has been done across the top of all the pages of the passport.
Open the passport from the back and turn back one page. This is page 31 of the passport, the page containing the holder's details. It is very stiff as it has the security chip in it and is laminated.
Turn the passport so you are looking at it in a landscape position. The passport number can be found in the top right hand corner to the right of the code and directly beneath the words 'Northern Ireland.'
On a passport issued before October 2010, the page containing the holder's details is on the inside back cover. According to the Identity and Passport Service the only ways to legally obtain the number of a British passport is by looking at your own or if you are an official who has access to one. There is no legal way of finding a British passport number otherwise.