Am new here and this is my first post, so hope I do it right, please feel free to correct me if not

I have just received a wedding certificate for ancestors from 1963 that states they were married in the register office, Liverpool South.
Where would this be? I thought there was only one register office in Liverpool. I know there was one in Mount Pleasant in the 60's, so could this be it?
Also, I have a birth certificate from the 1920's that states the child was born in Garston, u.d. What does the u.d. mean?
I no longer live in Liverpool - left several years ago - so am unable to do any local searching etc.
Many thanks for your time.
Felix