What
@Murdoch is getting at is your electrician can't just say "he needs to replace all of them before he can issue the certificate" unless he specifies what electrical regulation the original lights are not complying with. This will have a regulation number.
As
@Murdoch says, email your electrician and get them to explain why they all need replacing and what electrical regulation they are going against. State that you would like the reply in an email so that you have written proof of this conversation.