I’m sorry if I appear argumentative, but there is a difference between photocopies and Certified copies.
You only get photocopies from the GRO if you order a cheap PDF copy, Paper ones are Certified copies and that is all you ever get, even when you register a birth or death. I have my original certificate issued when my father registered my birth, that clearly states it is a Certified copy, that was used for my first passport application. When we registered the birth of our eldest son, what the Registrar handed us was a Certified copy.
It is the same with marriage certificates, the one you are given is a Certified copy.
If you need further proof, check out the Government website on how to order a passport, it clearly states if you don’t have your original birth certificate, you need an official copy, it goes on to tell you how to apply for a copy from the GRO.
Addendum
The requirements changed for people born after January 1st 1983.