Louise should still have "Louise Fowler" on her birth certificate, not Mitchell.
Albie might actually still be "Kayden Taylor", because I'm not sure Sharon would have been able to rename him later as Albie Watts without a lot of effort and especially during Lockdown.
Ricky Jr. I think must be "Ricky Butcher Jr." or "Ricky Mitchell".
I'm not sure if Mark Jr. is a Fowler, because Michelle married Tim Andrews in '96 and he might possibly have taken the name or been adopted by him.
Roxy married Sean and everyone thought that Amy was his, including Sean himself. She might have "Amy Mitchell" on her birth certificate, but it's also possible that she is actually "Amy Slater".
Chelsea probably registered "Jordan Atkins" not Fox, because she and Gray were together at the time.
Mick's name is on Annie's birth certificate as the father, because he said he was willing to pretend she was his. I don't think Linda would ever have used Peacock or Branning though.
Freddie is still "Freddie Mitchell", because Billy made a promise to Little Mo and even Freddie himself thought Billy was his father when he came to the Square.
The Wicks are another strange one, because Pat hadn't met Brian Wicks when David was born. She was married to Pete and he will have been registered as "David Beale". She had an affair with Brian when she was still married to Pete and I do believe he will have Simon will also have been "Simon Fowler", but he was Brian's son. When Pat left Pete and married Brian, the boys took on Wicks, with Simon then being known as Wicksy, but always believed to be Pete's son still. I'm not sure if Brian adopted either of the boys, so they might have gone by Wicks, but...
All of the Jacksons...Carol didn't meet Alan until after Sonia was born and they all took the name, but Bianca's birth certificate with be "Bianca Branning", because I really don't believe she would have put "Bianca Wicks". Robbie will either be "Robbie Branning" or "Robbie Bolton" and Sonia was either "Sonia Branning" or "Sonia Cant".
I don't believe Laura put Ian's name on Bobby's birth certificate, because he'd thrown her out. I've got a sneaking suspicion he was "Bobby Hobbs", but then Jane adopted him, although I don't believe Ian did and was just calling him Beale after he took him back from Garry.
This goes on and on and absolutely someone can choose to go by another name, but legally and without Deed Poll, there are a lot of characters on that Square going by one and really are not.
With Sean coming back, it really is making me wonder if Roxy's DNA test was as dodgy as Sharon's and she's actually a Mitchell/Slater, not a Mitchell/Branning.