Your experience was my experience. There were two annoyances:
It's not entirely clear which photos to get using a Photo Me booth. I got the wrong set of photos and had to reshoot.
That is because when you're doing the American passport photo option, the Photo Me booth gives you a constant string of warnings saying that it's not compliant with the passport. But of course, it's referring to the UK passport, not the American.
So when I printed out the first set of photos, I saw that they were clearly too small, so I had to go back into the booth for another round of photos.
The other annoyance was the fee that you have to pay the post office on top of the passport fee. So it's not enough to bring the exact amount for the renewal. There's the post office fee on top of that, which is something like under £10 if I remember correctly.
Thankfully, once I dropped the envelope into the appropriate receptacle, I had no further issues and did get the passport in less than 3 weeks.
The one depressing part of this procedure is seeing the expiration date of the new passport and knowing you will be 10 years older at that time...