I'd add that the form has loads of pages but for someone who was born in the UK and just getting a renewal, it's about a 15 minute job. The most important thing is to secure an early morning appointment IMO.
You go to a post office to pick up a form, go online and book an appointment (usually in 2 or 3 days time), take the form and photo with you. Given that there are photo booths in the building I presume you have the opportunity to fix errors there and then.
I've used it earlier this year and the service is excellent. Handy that it's next to Victoria Station, I got an early appointment that lasted all of 5 minutes and 4 hours later picked up my passport.