If you're confident in your picture-taking ability you can also take your own pictures and resize them on
www.epassportphoto.com/. They have different countries (and therefore different dimensions) that you can select from a dropdown menu. After you upload and fit your digital picture to the dimensions on their website they'll give you a downloadable link to a sheet of 6 passport size photos, which you can then print through any regular digital photo printing service, like Boots or Snapfish. It ends up only costing as much as one regular sized photo print (about 10 to 25p). Their website even has all of the guidelines you'll need for how to take your photo (such as the background color, what not to wear on your face, etc.)
If you're at all concerned I'd recommend getting it done professionally to eliminate the stress, but I used epassport when I needed a UK-sized photo for my visa app when I was in the U.S. and they were fine, and DH and I have carried on using them for driving licenses and such.