I was adopted by my step-father when I was three & he married mum but the judge had not signed the papers so when I got married 33 years ago there was a problem as my birth certificate was also lost when the records office in London had a fire & mum never had a copy. At that time the register put my original surname followed by "now known as", to overcome the problem for the marriage certificate.
This was not considered a legal document when I went for my passport but they accepted a letter from my doctor as proof. Maybe this could help you.