I've never used the Post Office service, and my last two renewals have been done with the "one day" service, so I've just gone into the passport office next to Victoria Station, queued up for a while, spent a few minutes at the counter, and then had to wait a few hours for the passport to be printed.
On both occasions I had no problem with the form, but it does come with quite a lot of information about exactly what you should and shouldn't do. Read things carefully, and you should be fine.
I've also know people use the Post Office service, and still have the form rejected, because the Post Office counter bod didn't notice a mistake. I think they could have gotten their money back from the Post Office, but obviously not all the hassle involved, which was what they were trying to avoid by using that facility.