So, in my case, the main reason to use an NCS is that my husband is a champion worrier and he'd just spend months fretting that I forgot something if I did this by mail.
That said, the web page says
You must submit either original documents, or a fully certified photocopy of your documents.
After the nightmare of the ILR application, I think none of us would want to send originals. But what constitutes a "fully certified photocopy" in this case? I have evidently not lived here long enough to understand what is meant by this term, and whether it's really equivalent at all to "notarized". (And what fraction of £40 it would cost to get that done.)