Passports are annoyingly expensive. They're not oh-so dear, but it's more than you'd think they would be.
And in the UK, if you're having your first ever passport over the age of 16, you have to go to a passport interview, where you're asked questions about your family, address, telephone numbers, education, etc.
Things they can use to certify that you are who you say you are.
The interview isn't the troublesome thing, it's having to journey to the location for the interview, which may be several hours away. So you have to afford to travel there, and do it while you're not working, or get time off of work.