Royal Mail counter staff aren't going to be typing in the data for proof of posting certificates at the counter any more as of January 1st. Instead they want you to print and write out a proof of posting form which gets stamped when you had in the package(s) at the post office. I can actually see the sense of this, doing the proof of posting thing takes at least 20-30 seconds per letter, which is not fun if you're stuck in the queue behind the person posting 20 letters. The form's here as a PDF:
There's also a form for signed-for mail (and special delivery etc.) accessed from the same page, not sure how this helps since they will still have to type the details into the system - I think that one's intended for multiple packages only, but I could be wrong.
Not sure how either will work at post offices with self service machines where you type the customer's address etc. into the system.