I am in the process of selling, and have found that our new kitchen and bathroom, installed by a previous occupant, should have had some form of consent.
This is the reply I got when I asked exactly your question:
“Thank you for your email in regards to works which have been carried out retrospectively. I can confirm that a formal Retrospective licence for alterations would not be required for the works you describe. Please liaise direct with your ward manager, [name] who I have copied into this email. If the ward manager is content with the works carried out then they will issue retrospective written consent in the form of a letter.”
Do not follow the process you are directed to on the Camden website – it is an online enquiry form but no one seems to read them or answer. I wrote an email to email@example.com and got a fairly quick response.
Having said all that, I haven’t yet had a reply from the ward manager!