Don't worry. Go to somewhere like Timpsons - and other places, I am sure other posters will name them. Timpsons take the photo and send it automatically to the passport office, then give you a number to type into the online form.
I replaced my passport last year and it was an absolute doddle, apart from getting the photo taken, which I did when doing my weekly shop, it took about 10 minutes. I have had a passport since I was 15 and it has never been so easy
One word of warning make absolutely sure you are directly through to the Passport Office (the web address should end .gov. There are a number of commercial sites around with very similar names, their aim is to get you to use their site and then charge you an 'agency' fee on top of the PO fee for acting as your agent to make your application. However their sites address will not end .gov.