All UK birth certificates are certified copies. The "original" is handwritten in a large book by the birth registrar when you go to register the birth.
You don't necessarily need to get more copies from the GRO, if you know which council the person was born in you can go directly to them. It may even be cheaper as they have less searching to do.
Births are public information so anyone can buy the UK birth certificate of anyone else, and you can view the full records at some public libraries.
In 2023 I wanted to renew my British passport, but the option to send photocopies of each page of your Aus passport did not exist. As I didn't want to post my Aus passport to HMPO, I booked a premium in-person renewal when I was in the UK for a month.
The staff member said they ask for all foreign passports because HMPO has a policy that the name on a British passport must exactly match the name in all other countries' passports. She looked at every page of my Aus passport then took a black and white photocopy of the photo page only.
My new British passport was sent the next day by secure delivery. 1 week later my old British passport arrived by normal post, loose in an envelope. 2 weeks later the b&w photocopy of my Aus passport photo page arrived by normal post.
During the appointment I had asked why they needed to look at all my travel stamps on every page, and she said she wasn't looking at the stamps but only for anything that might indicate that I changed my name. On the HMPO website you can find a long flowchart they must follow to decide whether different names are acceptable, when people claim it is impossible to change the name in the non-UK passport.